Successful online communities need to be easy to use. No one is going to use a really complicated online community when there are other easy ones to use out there, they also need to be accessible easily. While using an online community someone needs to be able to do whatever he wants, socialize, find interesting things etc. easily and efficiently a point older communities did not really grasp so people stopped using them when something easier to use came out, that being Facebook.
Right now all of the communities that are actually being used are easy to access, use and share so people will chose them over the older ones and possibly new ones coming out without the efficiency the already existing ones provide.
The point I’m trying to make is that an online network needs to allow a user to be able to do whatever (s)he wants with ease and without much hassle if it’s going to be successful.