The big problem with Facebook is that their business isn't defensible in the long term. There have been lots of other social networks come along in the past (AOL, classmates, Friendster, Myspace). The prior ones are all in shambles now, that people have moved on to a new place.
I don't see why another network can't come along in a year or two, and start stealing away visitors from them.
Compare a few publicly traded companies in the range of 50 to 70 billion dollars market cap (all seem to have a pretty strong 'durable competitive advantage', as Warren Buffett notes):
Comcast (63B) - millions of subscribers for phone, internet, and cable services, paying money every month for services.
Amex (55B) - receives a chunk of every payment that their millions of card holders make. Card is accepted at stores and websites around the world.
3M (62B) - sell 20+ billion dollars of "diversified technology" products a year.
I don't see why another network can't come along in a year or two, and start stealing away visitors from them.
Compare a few publicly traded companies in the range of 50 to 70 billion dollars market cap (all seem to have a pretty strong 'durable competitive advantage', as Warren Buffett notes):
Comcast (63B) - millions of subscribers for phone, internet, and cable services, paying money every month for services.
Amex (55B) - receives a chunk of every payment that their millions of card holders make. Card is accepted at stores and websites around the world.
3M (62B) - sell 20+ billion dollars of "diversified technology" products a year.