Games like Farmville are a fad, like Rubix cube and Cabbage Patch Kids. That's not to say that casual games won't continue to an important segment of gaming, just that they will not provide an income stream that justifies a 50 billion company valuation. If it wasn't for the fact that "the market can stay irrational longer than you can stay solvent", I would short a Facebook IPO in a heartbeat.

Facebook's core value is that (nearly) everyone is on it and it is considered to be "cool". The cool aspect will not last. I'd be amused to hear any arguments as to how it possibly can. Young people will move on to something else.

I also think it's very likely that their power as a centralized manager of personal contacts will be eroded over time. Really, it's not that hard of a problem and some other applications will end up being compelling enough for people to switch. They will try to prevent it (e.g. like trying to prevent GMail from exporting email addresses) but ultimately will fail.

The history of the Internet is filled with the corpses of technologies that were supposed to be "the future". Who remembers PointCast? How about Orkut? For a while RSS was touted as the solution to all of mankind's problems. ;-)