You're wrong now. FB's market share is almost certainly below 50%. They're a tad larger than MySpace, and there's also Hi5, Bebo, Orkut, Ning, Friendster, etc.

And their market share in the developed world isn't even in first place, and they're not even growing significantly faster than MySpace there.

Also, Google makes more per pageview by far than their 2nd place competitor, Facebook makes substantially less than MySpace. If anything in the social networking wars, MySpace is Google.