Sure. Facebook could be "MySpaced", too. The whole point of what happened to MySpace is that it was hard to figure out what it would take to disrupt them until it happened.
1. Pick a niche (college kids)
2. Build what they want
3. Let it trickle down (high school) or up (grandma) or out (businesses) to gain mass.
So while we can't pick the winner, we know what the winner is going to do.
I think the problem here is that when you start big and work backwards it's hard to build exactly what a particular niche wants. For example, starting backwards you never would have built the perfect tool for bands (myspace) or college kids (facebook) because you want it to have mass appeal.