I'm going put put my two cents in, and say that a platform capable of replacing Facebook for discussion and community on the web has already been created.
It's called Reddit.
Bear with me here, I realize this is probably an unpopular opinion.
On Reddit, all content is open -- I don't think I've ever seen a 'closed' sub-reddit available only to members. I'm not sure it's even possible.
Yet, in spite of all communities being effectively open, most sub-reddits have a strong sense of community, and are very active. Personally, I really enjoy proggit and r/math for this reason -- active discussion, community moderation, and a strong sense of community.
It's called Reddit.
Bear with me here, I realize this is probably an unpopular opinion.
On Reddit, all content is open -- I don't think I've ever seen a 'closed' sub-reddit available only to members. I'm not sure it's even possible.
Yet, in spite of all communities being effectively open, most sub-reddits have a strong sense of community, and are very active. Personally, I really enjoy proggit and r/math for this reason -- active discussion, community moderation, and a strong sense of community.