LocalGroup... how they make their money isn't the point...
FaceBook is popular because... it's popular. Once a significant body of it's community move on to The Next Big Thing then the whole thing collapses in on itself.
And this is already starting to happen, most crucially for FB among it's prime demographic - teens and young adults. So yes, while they can still massage the numbers to give the impression of growth, now would be a good time to go to the market, before the market realises that FB's time in the spotlight is starting to - if not pass - at least peak.
FB's infrastructure costs must be phenomenal... when user numbers start to fall (and they will) the ad revenue will dry up at a much faster rate. The fees from virtual currencies will similarly dry up once the providers of those games establish enough of a presence to create their own economic communities separate from the FB platform. You may have read that Zynga have already pulled the plug on a number of "FaceBook" games, and are now starting to create their own eco-system separate to the FB one that gave them such an important "leg-up" to get started.
There will come a point when FB will be forced to consider subscription fees for it's users just to cover it's costs, and once that happens a new FaceBook will spring up to start the cycle all over again.
(and by that time Google+ will be very well placed to pick up the pieces)
The point being, that those standing to make most from an IPO will know all of this already and perhaps even have had a red letter date circled in their calendar for when this all has to happen for some time already.
It's time for them to cash out.
The only question for you, as a potential investor, is do you want to provide some of the glittery lining for Zuckerberg's Golden Parachute ?