There are 101 ways to look at API and to provide API. I would wager that Facebook would be just fine without it. In effect API is a form of surrender - you can't do it on your own, so you provide tools to "connect" with other software.

What's more important, IMO is a good SDK. Think of the Apple's success - staying as closed as it gets they keep expanding. Connections, integrations and bridging (read API) creates more problems than it solves.

And no, I don't thins Dolphin isn't see more comparable to Facebook. Its "marketshare" is much smaller, but it's different in a good way. It's distributed and unlimited in potential. Also, it's liberal.

The only challenges that I can see are: ease of deployment and updates; initial costs; code quality; UI intuitiveness, etc. - all of them are solvable bits that we'll gradually overcome, while Facebook fatigue and some tech expertise build up and prepare the grounds for the big new game.