Is it really sustainable? I guess at this point that's as much a question about the future of world culture as anything. Are we going to continue feeding their machine?
It's likely to continue to be the only social network in the west that is truly wide, spanning almost everyone. That will probably be worth $40+ billion in ad sales in two or three years.
15 years ago, who thought a search engine could scale to $80 billion in advertising? Essentially nobody. Microsoft at the time had $23 billion in sales. You would have been laughed off the planet if you forecast Google's search engine to be worth so much.