But I guess that if Blogger would be "open", so that other search engines could also index those blogs, then this whole scenario would not work (malignant search engines could expose everything to everybody). So my suspicion is that it would actually be quite hard (or at least require more research) to make an "open" version of facebook.
To me, Twitter benefits from being public discourse and an open forum for replies. On the other side of the coin, Facebook benefits from being a private forum (as most people use it).
Solutions that don't facilitate these use cases aren't going to be successful. Understand why people chose the services they did (discounting network effect) and then try and build something to compete.
lol u srs? j/k <3