The momentum (network effect) makes Facebook extremely useful. Like most big institutions, it does have it's problems. If an alternative could get going, it's always good to have choices.
For an alternative to get off the ground, I think it would have to go the opposite direction from privacy. On a new platform, one would need to be able to brows freely without having to be in the friends list of the people they are browsing. The posts and topics would need to be searchable, both inside the platform and in Google. I think posts and threads would need to be visible on the open web to gain publicity. It would take this, at least for a while, to get the ball rolling; so to speak.
This would be the opposite of what some people want when they want more privacy. To start and to get the ball rolling, it would need to be open; in my opinion at least. It would kind of be like going back to the web, before Facebook, when people had their various pages and blogs available. Today, few people think to go to my blog anymore. It does get some traffic and searches, but most of the interactivity is on Facebook. That's why I use Facebook.
In the 1990's, they had a term called "push technology." That was when information, on a web site, would be pushed out to a list of subscribers; like being in a newsgroup or on a mailing list. Facebook takes care of that now. On a new platform, people could still sign up to friends lists to subscribe, but they would also have to be able to brows the content without having to be subscribed, or in the friends list, of the person posting. To get the ball rolling, it would have to accommodate the lurkers.