Being a Google+ user, I'm not really familiar with it, but how about Quora?

And mainstream users don't really see the problem with giants or tracking, they see how Google's search results page is getting more and more complicated with features they don't need. The original reason to use Google instead of, for example, Yahoo!, was twofold: the website looked very clean and the results were good. Now DDG's results are nearly matching Google, and they provide a much better interface plus a few features (mostly for developers, like MDN integration). What happens? People switch.

If Facebook works good for users, I guess they'll stick.

I'd say that would be a major purpose re-definition for Quora, as it's not primarily a social network.

Facebook is overwhelming used to share photos, and the photo sharing apps are the ones challenging it (instagram, tumblr,etc.)

Whatever happened to Diaspora? Did that fizzle out?

They're still on invite-only. I remember them having some problems, but I think it's also taking a long time because their idea is so damned ambitious...