What keeps the stock up is the idea that one day they may do something other than ads, and sell something to their user base.
I don't know if there is an example of a public internet company that was able to transition from being an ad-base business into selling stuff. The closest may be LinkedIn, who's selling recruiter tools and premium memberships. However, LinkedIn had been doing that before the IPO, as a relatively small company.
I'm skeptical that Facebook can pull this off in a reasonable time frame, but shareholders who are psychologically anchored to the IPO price are probably more optimistic.