Connected to an email service.
With some automated responses (like) and mass mailing features.
Connected to some games
All held together by exclusivity That is, they won't let you someone's blog, email them, or get emails, unless you join them.
Well, I did leave some extra stuff out - but basically the other stuff is all the privacy killing back office things that no users wants - i.e. the ability to tag other people's photos, the ability to track people viewing, etc. etc.
If you make a distributed version of it, it's called THE INTERNET.
P.S. It already exists. Frankly, the entire thing is just a simplified way for non technical people to get involved on the internet. Not everyone realizes how useful a blog, mass mailings, etc. are so they packaged them up as a "Social Network" and suddenly people that never heard of a blog are blogging.