I realize that we all have strong opinions about the topic of software patents, but in an effort to radiate more light and less heat, I think it is important to understand the official Twitter position on this topic as well.
Introducing the Innovator's Patent Agreement
Note: Emphasis is mine.
"We will implement the IPA later this year, and it will apply to all patents issued to our engineers, both past and present." 
"The Innovators Patent Agreement (IPA) is a new way to do patent assignment that keeps control in the hands of engineers and designers. It is a commitment from a company to its employees that patents can only be used for defensive purposes. The company will not use the patents in offensive litigation without the permission of the inventors. This control flows with the patents, so if the company sells the patents to others, the assignee can only use the patents as the inventor intended." 
Contracts require (amongst other things) counter-parties who also sign and the exchange of goods or services between the parties ("consideration"), neither of which unilaterally signing some random document has.
In law, it is very difficult for individuals to bind their future selves. You generally need another party to create obligations that they can enforce against you. So for example counter-parties to a contract, trustees and beneficiaries, employers and employees, fiduciary agents and their clients etc etc. All of these require distinct legal persons to work, if you you combine them into a single legal person the obligations evaporate.
Because when it's you vs you the law pretty sensibly throws its hands in the air and says "well what the hell do you want the courts to do? Hop in a time machine and drag past you into court?"
I haven't read the patent. But it seems to me that prior art would be CompuServe's CB Simulator -- from the early 1980s. That's what I thought of when I first encountered Twitter.
The independent claims all explicitly codify the follower/following mechanic. So if you want to show prior art, you'll have to find something where a user was subscribing to receive broadcast messages from other explicitly-defined users, as opposed to subscribing to a channel, group, mailing list, etc.
The heaps of similar methods that don't include follower/following is a good reminder of how easy it is to avoid infringing on this patent: just skip the follower/following mechanic.
I would be tempted to say that automated agents would obviously be prior art, but inventions that just automate long-standing physical workflows in a straightforward way (do it on a computer) have been found patentable more than a few times.
So I wouldn't be optimistic that the reverse isn't also true.
I'm not a patent lawyer or anything, but I don't think that is the important part. It looks to me like what they're really looking to patent is cross-media communication — Bob submits a tweet via the website, Chris gets it texted to his phone and Danielle sees it pop up via Growl notification from her client app.
LiveJournal doesn't route messages. It performs database queries which renders to HTML. As a result twitter will broadcast what you say to all of your followers, immediately, to their cell phones. This isn't something LiveJournal's architecture supported.
Is this because someone forgot to patent the cc feature of email? We've been doing similar point to multi-medium translation of messages for decades. Email to email+text pager for doctors in hospitals. Surely there's a strong prior art case that invalidates this, but this is also obvious enough that it really shouldn't get through in the first place.
If the guy that invented sms patented 140 char messages there would be no twitter. Or irc messages, or smoke signals, or X where one people sends a text message to others.
Patents are a bureocratic perversion that do not acomplish any progress to humanity, they just perpetuate the bureocratic system itself.
Many of these humans are caught within a system which incentivizes personal interests over those of society. Those in higher power are aware of this, but too cowardly to do anything about it, sadly, and the lower amongst us are forced to at least protect themselves from a broken system. But the thing is, the more they protect themselves, the more this corrupt system becomes reinforced and innovation becomes even more endangered. We need to wake up and say enough is enough already. Do we really want this paralyzing system to dictate us and future humanity?
The patents involves routing messages with emphasis on followers. Everything previous I'm aware of has been multicast based or address based, like email or mailing lists. Most internet and application-layer protocols are destination based. Where is a protocol that says "Route this message based on who is interested in the source."? All I find is "send this message there, based on X, or, send that message here, based on Y"; both destination based. Would be interesting to know or hear of past protocols which are more similar to twitter's patent.
Everything previous I'm aware of has been multicast based or address based, like email or mailing lists.
As others remarked, IRC is follower-based. You follow a bunch of channels by sending subscription message (JOIN #channel-name) to central authority (IRC server). The server keeps track of who follows which channel, and brokers any PRIVMSG #channel message messages to the followers, until the person does PART #channel-name or disconnects. Aside of that, a direct person-to-person mechanism exists just as well, also via the PRIVMSG command.
The USER nickname command provides, among other, list of (public) channels the named person follows, which some IRC clients use to allow user to follow the same channels.
Yes, there is a difference between an IRC channel and a singular persona, but it is not as clear-cut as it may seem on its face. On one hand, some channels are muted, and only admins, or just a single admin, can post. On the other, Twitter handles often stand for multiple persons, speaking for one organization, team etc.
* * *
As an IRC user, I am ashamed and appalled of how low our industry has fallen -- somebody applies for a patent for what (after cursory read seems to me) a copy of well-known and long-established functionality.
IRC has no such long-established functionality. You can MSG a #channel or a user and that's it. User's can't follow each other and there are no lists. These followers and lists are how twitter routes messages, not a channel.
Such a simple difference makes all the difference. In this case, enough to file a patent.
If they didn't file a patent someone else would have.
This is almost definitely a direct result of the recent legislation that changed patents from a 'first to invent' to 'first to file' system. Basically, Twitter had to file a patent on twitter-like services, or someone else could come along, patent it, and force Twitter to 'license' the tech.
That is not at all how "first to file" works. You still need to be presenting a novel invention that doesn't have prior art.
First to file drastically reduces the number and complexity of patent disputes and it improves the stability of a patent once acquired.
And there's always been a de facto paperwork requirement under first to invent anyhow, it's just that you had to keep nauseatingly detailed "inventor's logs" just in case somebody sued you. And even then you could be bankrupted in court by someone running a frivolous interference case.
First to file is a net win. That's why everyone else on Earth has been using it.
If you don't care about your websites(eg. Hobby projects, projects that aren't doing well) or care a lot(eg. Your site is the next facebook) approach a Patent lawyer and have your site reviewed. If he/she finds something amiss, approach twitter and ask them if they are ok with your website. If your site infringes the patent and you don't care about your site and you do this, you end up helping people understand where twitter stands on the issue of patent abuse and trolling. If you care a lot about your website and you do this, you are safeguarding your interests.
However, if you care just enough for your website(eg. Your site makes enough money to pay for itself and then some), I would advice against approaching twitter. Chances are twitters lawyers will ask you to shut your project, if you approach. And if you don't, chances are your project might be small enough and remain unharmed(security by obscurity?) if twitter goes a-trolling.
I'm the in the middle of patenting an actual protocol (unrelated). In case you're wondering, yes: patentese is always vague and difficult to follow. I wrote a few thousand words to describe my protocol to the level you'd find in an RFC, and it blows out to tens of thousands of words of dense verbiage and a bunch of diagrams with hundreds of reference numbers.
There is prior art for this. I've seen forums that permit turning on notices for all posts by specific users, which in itself looks to satisfy every claim, since they also include a "what's new" feed which is basically just a dumping of posts anywhere on the site into 1 place. They behave like a miniature twitter in that regard.
IRC clients also notify the user when messages targeting them (via your handle, similar to @handle) are received, and I've seen scripts that pull comments from specific users from any open channels, and the opposite (namely blocking users) has been present in countless applications, which indicates the concept of paying special attention to a specific user isn't a new one.
On top of this, I don't feel like there's anything that's innovative at all in this patent. Since when did the innovation step stop being a requirement?
At least they have... what? A shill-ish, apologetic article on Wired that doesn't offer anything except "we pinky promise not to troll"? There is still absolutely nothing stopping them from using this however they please.
IMNAL, but, AFAIK, you can infringe individual claims. The reason patents are written this way is generally to pack together all related things, claims, into a single patent rather than file several, 22 in this case, patents.
American companies had patented many famous varieties of rice and other corp. Not to mention many medicinal plants that is being used in India not for centuries but millennia. USPO granted them all that. Luckily Indian Govt showed some ball at that time(which is remarkably rare), and they directly told the patent holders to stuff it up their ae.
Matter was settled.
Can't sth like that happen in computers and Internet?
(BTW, it seems either USPO is very corrupt or very negligent. Or maybe they don't give a damn!).
I doubt very much that the USPTO is getting kickbacks to pass patents through. As I understand it, "number of patents granted" is the metric they use for success, which puts a lot of pressure on the patent examiners at the bottom rung to do very basic research before pushing things through.
Basically, there is no incentive for them to do a diligent job, so they don't.