1) When it started, there were already many social networks
2) Because it positioned itself as a social network for college students, it got the early growth and adoption any "social" website needs to survive
3) They've grown REALLY fast since then, and have started to really lock in the users
4) It's not unthinkable for them to have a billion users. Once they have all the users they want, they can pick and choose their business model.
5) Mark Zuckerberg can't be removed from the CEO position, and probably holds more than 20% of the company
6) Facebook is hiring top talent, on par with Google and probably better than Microsoft.
These things don't just happen out of the blue. For someone who has consolidated power in a company that could rival Google, I don't think Zuckerberg gets enough credit. If I were pg, I would have put him in my top 5.