Growing to a billion users is harder than you think. There were many other social networks, including ones started at other top schools, but only one survived, and it was because they understood what makes a site like fb work.
Edit: also half the problem is caused by assuming 1 billion users translates to anything meaningful. Apart from being an impressive number in its own right, what exactly does it mean? Facebook is free to use and helps satisfy basic human social desires in a digital way. Great but would they have 1 billion users if it wasn't free? No.
If I could distribute beer for free and and offered every male on the planet over the age of 18 a free keg, I'd probably have a billion takers.
Which internet company does, in your judgement?
$100,000,000,000 is a VERY big number. It feels almost absurd (to me at least) that a single company can create that much "value" in less than 8 years. But maybe I'm the one with the problem and I just can't get my head around "web scale" economics.
When I apply that logic to FB's 100bn valuation, it makes sense. Doesn't make us all look any less stupid though.