Facebook is filled with smart people. They understand the trends better than we do. As you'll note, 4 of their top 5 Risk Factors relate to the web-to-mobile shift and the problems inherent with that.
From the filing:
#2 - "We generate a substantial majority of our revenue from advertising. The loss of advertisers, or reduction in spending by advertisers with Facebook, could seriously harm our business;"
#3 - "Growth in use of Facebook through our mobile products, where we do not currently display ads, as a substitute for use on personal computers may negatively affect our revenue and financial results;"
#4 - "Facebook user growth and engagement on mobile devices depend upon effective operation with mobile operating systems, networks, and standards that we do not control;"
#5 - "We may not be successful in our efforts to grow and further monetize the Facebook Platform;"
I think this is correct, and it explains why the IPO exit is happening now. Most of the insane growth (in profit as well as valuation) has already been squeezed out.
I would amend that to say: most of the easy growth has been squeezed out. But there's still a huge opportunity for Facebook to grow revenue/profit if they switch to using personal data to make well-directed ads they can push to sites using Facebook Connect and/or if they can roll out a solid first-party Facebook-phone. 
Those are both far trickier than "grow Facebook", so it does make sense to capture some returns now. But I think it would be short sighted to write their future prospects off entirely.
 They could quite possibly fork Android and do to Google in the phone market what Amazon is doing to the tablet market. And they'd be crazy not to try.
Facebook has (wisely) focused almost entirely on user growth and retention over the last few years. That it stumbled into being an attractive display-advertising platform was almost inevitable, and required little effort. The real money lies on the distributed mobile and web ecosystem, into which Facebook has been diligently inserting itself as a de facto credential and backend. (Think of Facebook as the "Intel Inside" of the entire content web and mobile content web, and you get the idea).
Facebook has established an impressive beachead with FB Connect, and from there, it's only a matter of time before it can monetize its FB Connect partners through contextual and interest-based advertising services. Google should be very concerned -- and, to that end, it would not surprise me in the least if the threat of FB Connect to AdSense (and, to some extent, AdWords) was a significant factor in the race to develop Google+.
No, hitting the investor limit means only that you have filing obligations with the SEC. There is no obligation to trade your stock publicly.
Interesting. While the other points are gloomy-doomy and a bit vague, this one lends itself to a concrete solution, doesn't it?
Edit: Ooh i get it.
So actually I don't worry about either company's continued success. Both Google and Facebook are here to stay and grow even bigger.
Let's not get carried away, this is not true at all. Even outside of tech circles, there are plenty of people that have no interest in what Facebook offers.
Phone numbers are definitely obsolete.
Phone numbers are abstracted, but they are definitely not obsolete.
Semantically, choosing "obsolete" might have been incorrect, but I think the point stands. IP address and phone numbers don't matter anymore to the end users.
Since a lot of mobile apps have some type of Facebook integration (ie: a Facebook app id), Facebook has a good idea of who has what apps installed and they can serve up apps that are relevant to the user both on the interest graph and the data of what other mobile apps they have installed.
Wouldn't be surprised to see them move into mobile advertising hard in 2012/2013.
* Disclaimer: This is all speculation, no insider information was divulged
They talk much about Pictures, but most of the people i know don't post any. The big success of Facebook is the E-Mail IM replacement for young people, but it is very difficult to advertise on that. Who made ever money on IM and Mail? ICQ and AIM had impressive numbers, but were never a big success financially.
I know also people who share them, but max every week and probably just 5 friend really look at them. When you look at pictures, how can you concentrate on ads? They are not even in the middle like Youtube's. Pictures on mobile reduces the space for ads even further and you will always be uglier than every competitor.
I'm not really sure that applies universally. The majority of my stream post pics from mobile (either directly, thru instagram, etc.).
>The big success of Facebook is the E-Mail IM replacement for young people...
In mobile? I don't see that happening either, as most too used to texting/iMessage/BBM
Where I do see a lot of usage is in-stream. If they can figure out how to serve up ads within the timeline without being too intrusive then that would be the way to do it in mobile. Tho that new timeline crashes more often than it works in mobile (at least in the iOS version)
What is the benefit of looking at someones timeline and how often will you do it? Facebook needs incredible engagement (what it has right now on the desktop) and space do display ads. Youtube has the content, but what brings people back to FB? Don't forget all these social Startups will hurt Facebook on engagement.
Also, I believe stats will show that engagement is rising rapidly on mobile, and that's what keep people logged on FB.