A very, very good question.

A lot of people (Ian Lurie in particular) have been saying that Facebook's IPO is too late and should have been last year when they were incredibly overhyped. I have to agree. I think Google+ is proving a credible threat, and the investor excitement around Facebook just isn't what it once was. That said, they probably can still have a massively huge IPO that turns tons of employees into multi-millionaires, and that's a good thing for the startup ecosystem. The risk is that they fail to deliver on the promise of the IPO and it makes the IPO market as a whole even less appealing to companies doing significant, impressive revenue.