At the end of the day, sucessful business revolve around sustainable growth. Facebook can have one, but not both.
It can sustain its current business model, and keep its large user base for many years to come. But that will leave its annual revenue dwindling around the $5B mark, making its $100B+ market capitalisation hardly anything to write home about.
It could try to grow further, but with even more advertising required to do so, that will only serve to tarnish its current website model, and likely turn away many of its users.
Unlike Google which has many sectors of the website it can exploit, Facebook is limited by its own website and the loyalty of its userbase. That will bring upon the downfall of the stock in my opinion - there's just no plausible avenue for sustainable growth from where it already stands.
When the hype settles, I'm pretty sure this thing will take a leaf out of Microsoft's book and flatline in the $25-$30 range. There's alot of innovation behind both companies, but not enough for either to take the world by storm like Apple and Google have.
