The key to any social network making themselves “sticky” is that they need to create a unique product. Alternatively, they need to isolate their user group so that you can only access certain people using this one specific product. A social network or Web 2.0 tool may do both of these things, and those are perhaps the most successful. Anyone can have a forum on a specific topic that wants users to come back to get the right content, but without either making it unique or trapping those users, anyone else could do it, too.
Take Facebook for example. They grew in popularity and stickiness by first creating a social network for college students. They succeeded in being the “place” to be for college kids, and, by limiting their membership created a feeling of uniqueness around their product. By college kids, for college kids. Even though that no longer holds, the unique character of the facebook brand, combined with their limited data portability (try pushing your facebook status updates to Twitter, instead of the other way around…), makes them a super sticky product. I can’t think of anywhere else that I can check up on the relationships of people I knew from fourth grade.
MySpace is another good example of this. The uniqueness of MySpace lies perhaps in its catering towards musical endeavors. Facebook is a personal brand, MySpace has found its niche in presenting musical content automatically, making it a commercial brand. What other product allows an automatic music player to bombard visitors against their will? Not saying it isn’t annoying, but it works. And it has stuck.
Twitter, too. Their uniqueness is in the format. “Microblogging” conforming to SMS standards so you can interact on the go. That data is portable, and you can find Twitterers elsewhere on the web: their homepages, blogs or social networks, but you can’t find that unique content presented in that way without interacting with their Twitter profile.