I have two major issues with their revenue model.
#1 - Half of the users are logging in monthly are via mobile and there is zero revenue stream from mobile. If they begin polluting the mobile segment with ads, due to the decreased screen real estate, I think it'll have a negative impact.
#2 - 11% of their current revenue stream comes from Zynga, which is online games played through FB. I don't know whether or not that is going to be sustainable.
Hot IPO's lately have fizzled (Groupon for example) so it'll be interesting to how the market reacts to this one. I think you'll see a lot of early buyers who short the stock before it starts its drop.