I always have a feeling that facebook is limiting entrepreneurs. You can't launch a social product today without making it facebook compliant and this means abiding by facebook's rules, and working within their structural limitations. These limitations they've imposed ensure that they keep their market share and prevent anyone else from really growing in the same space as them. But I don't think they can stay at the top forever. Demand for an open alternative is too high, and I'm really hoping (along with many others) that Diaspora can make some headway with this.
I know a guy who recently tried to build a social application, starting outside of Facebook. It was actually a very good idea and well implemented; unfortunately, the complete lack of connectivity to groups of people sealed its fate. By the time he got to integrating into Facebook Connect, he was almost out of money.
I think entrepreneurs wanting to build a social product are in better shape today because of Facebook and its built-in social network. But you are, of course, correct that building on somebody else's platform is always dangerous.
Lastly, "Open" is not really a feature to end-users. Diaspora will win if they can build compelling features that Facebook, as a result of its closed network, can't. Otherwise, I doubt highly it will be successful.
