Absolutely not. The whole nature of twitter is about its spontaneity and real-time nature. Can you imagine if someone had to go through the extra thought step of "which group should I send this to?" every time they sent out a tweet? It would ruin the service.
Also, a primary difference between Facebook (now Google+) and Twitter is that Twitter was assumed to be public by default. It is a public form of communication, and the Google+ style of sorting your messages into specific buckets of people doesn't fit it at all.
...and yet I know a ton of people who have fully-private Twitter accounts, every one of which who bemoan the fact that they don't /want/ to have a /fully/ private account, but that they are more comfortable sending an important subset of things they say to people they know, an are therefore stuck.
These people then end up having multiple Twitter accounts, which is a drastically more difficult "extra step" to perform when they go to post a new message: sufficiently so that a good number (most, in fact) of the people I know who do this simply don't bother, and /only/ keep the private account.
(Although, I personally am in this overall category, but chose to keep only the public one, and thereby simply "don't really use Twitter, as they apparently only care about weird totally-public PR garbage".)
If Twitter added the circles feature tomorrow, people who actually considered it a pain to think about simply wouldn't: they would ignore the box, and business would go on as usual. However, if they wanted the feature, they would probably start using Twitter a lot more; and, as they are no longer "rapidly" switching accounts before posting, that allocated time would be even more effective.
(Also, for the record, if Twitter did do this, it would be even extra awesome, as "listing" someone already provides all of the "circled" data model, meaning that a lot of people, like myself, for whom this feature would have immediate value, already have our followees organized usefully.)
Because all modern cell phones have Wi-Fi, and there are a lot of free hotspots around the city (here in Mexico), they use twitter as SMS... They log and send a message to their friends about the hour to hang out, and the place, and the chat over twitter... I'm nor sure if they know it's public, or the implications of that, but they truly do that...
I'm begging to understand twitter, as poor man's phone calls...
Google+ on Twitter would be more like a group DM. Twitter's mentions to users are public so mentions to lists could be to. I've got a list for people at work but to mention them at the moment I have to name them individually. It would be great (though perhaps open to abuse) if I could tweet at my lists, or even the lists of others. This could be implemented with another character like "/". E.g. "@/foo" to mention my foo list or "@bar/baz" to mention @bar's baz list.
The burden with Google+ circles, and Twitter if it followed suit, is that I have to manage what someone else is interested in hearing from me. That’s completely backwards for pubic sharing (which is unequivocally Twitter's focus), especially for people with a decent sized public following who they may not know very well.
What Twitter needs to build is a capability where I can advertise different topics that I talk about, and then people can selectively follow everything I tweet, or just follow tweets that I publish to a particular topic. In G+ terminology it would be like if they made my list of circles public, and then people added themselves to circles rather than me adding them.
It sounds like they really just need to let me search only my timeline for whatever I want. Interestingly, I can't even seem to do this with the advanced search (http://search.twitter.com/advanced)
Timeline restricted search might solve the problem, but there are plenty of topics where a simple keyword search may not cut it. Searching for programming would obviously include a wide array of tweets with a wide berth of keywords.
I don't think a naive keyword search would really solve the problem. I did consider at one point writing a Twitter client that would do simple Bayesian filtering across a list. So you would create a list, add whoever you want, and then flag messages as good/bad for that list, and it would start filtering.
Google's circle feature is incredibly useful to me. Not only will it come in handy separating my three professional spheres from each other, it is ideal for separating those from my family and friends. Each of these groups is interested in and privy to different types of posts from me, and finally, there is a way to share with all of them just what I want through a single account.
Twitter's simplicity is at it's core, however. I don't think Twitter could slip a system like this in very easily... nor can facebook.
Others have said this too, but it's not a matter of whether or not Facebook has that feature, it's the difference in defaults. I've been on Facebook for years and don't even know where to look for the "Friend Lists" --- and I'm a fairly technical guy who has tweaked all his Facebook privacy settings.
With Google+, the circles are core, not just some tack-on optional feature. The main way to share something on Google+ is by sharing it with circles.
To put it another way, the default on Facebook is to share with all "friends". The default on Google+ is to share with nobody - but the list of options starts with circles, and ends with the public. (Google+ also lets you include individuals in the list, but you have to type their name or email address, so I suspect that feature is little-used.)
I use friend lists and privacy settings on about a third of my Facebook posts. I can share something silly with everyone except my coworkers, something that's only useful to people local to me, or even share to the "public" with friends of friends or everyone. If I'm counting everything, I think it takes 6 clicks and picking a list name on Facebook vs. 2 clicks and picking a circle name on Google+. So the UX is a little harder. It wouldn't surprise me if Facebook makes some tweaks there.
I swear Facebook used to give you the option to view your incoming feed filtered by friend list but it's not there now. Maybe that can make a comeback.
Although I do use Twitter predominantly for work, seeing the odd glimpse of someone's personal life adds depth to my understanding of them. If it's too frequent, I unfollow.
If G+ is used 'correctly' then I'd be sad to miss those little insights in to my contacts other lives.
I am not aware, things like these could be patented ? That would be horrible to imagine. If that's the case, facebook may have patents on so many things, which google+ has copied.
Also, a primary difference between Facebook (now Google+) and Twitter is that Twitter was assumed to be public by default. It is a public form of communication, and the Google+ style of sorting your messages into specific buckets of people doesn't fit it at all.