As for what need Facebook meets... it's communication (and communing): people connecting with people. This is the basis of many technological innovations (telegraph, phone, internet; some would include trains, planes and automobiles as "communication" technologies). There is plenty of need left unsatisfied. The question is, has Facebook perfected what can be gotten out of the internet yet? If not, that means they have room to grow. (one day they'll be blindsided by a new technology that better satisfies that need; but that's a different issue). And, if there is room to grow, are they going to retain leadership? e.g., will Google overtake them? (google serves a different need of customers, so I think google will win there, yet not beat facebook at the need it serves).
Big issue: some people thought myspace would be long-lasting, yet facebook toppled it. Facebook is very aware of this fact. They have some idea about what would make them vulnerable, and, like Microsoft being very aware that they won out over IBM, they won't be complacent.
That said, IPOing before google+ hits its stride is good timing...