a). being birthed in Harvard (with all the clout, connections, mentoring and cachet that brings)
b). good timing (in terms of 2004 being long enough after the first dot com bust and before the crisis of 2008)
c). taking a fairly straight forward idea and improving on those before (live journal, myspace etc.)
The only cool thing about facebook as far as I'm concerned is the backend. Scaling a system to support 1 billion odd users is very cool and very interesting. Appreciating and respecting it on a technical level doesn't mean you have to appreciate Facebook "the whole", what it stands for or what it does. Also, just because it's an impressively engineered system doesn't mean it's worth 100+ billion dollars.
Edit: I guess I (and presumably some others) feel that the reward (a 100+ billion dollar valuation and 1000 millionaires created over night) doesn't immediately seem to translate to the effort spent getting there.