Network effect is always going to be a problem. But beyond that, Diaspora is pretty functional. And pretty easy to use - the docs make it look a lot more complicated than it is. It isn't as featureful (no galleries, no events/calendars). If all you want to do is post and share, and have the privacy of Google+'s circles, Diaspora will do it for you.
Oh is diaspora still alive? I would use it, but I'd guess I'd have even fewer friends there than on Google+