I am still totally amazed by what people share from their private life FOR FREE on Facebook. If you want to beat Facebook you only have to make a similar platform that actually pay people for posting their private stuff. Would be a nice startup :)
You're almost assuredly right that there are still opportunities out there in this space. I do think paying people to post would change the dynamic: it would change what and how often people would post. It's definitely an interesting problem!