You have to factor in whether or not you want developers to use your APIs in the future. If you do, and they have to guess whether or not other developers will build on it, they might decide not to go for it.

So maybe you don't really want to do APIs at all??

Deprecate them before you start! :-)

I was just writing about this re Dropbox. People always say about the idea I want to see them do "That's a third party opportunity." Well, I would never bet on a mission-critical feature that depended on the continuing support of an API by a platform vendor. What happens to the users when the API breaks? I might not have a lot of users from their point of view, but from my point of view, my community will die if they break the API.