All the most successful Web 2.0 sites are fun! Whether its Wikipedia having entries about your favorite hobbies and TV shows or its Facebook feeding you trivia about your friends' activities, there is a healthy entertainment factor in what can also be used for a specific productive purpose.
So the root challenge is not how to keep social networks separate so the oldsters don't harsh the mellow of the youngsters (although that may be a derivative challenge), it's how to make sure that both new and experienced users of Facebook of any age (or any social networking site or any Web 2.0 site for that matter) continue to find Facebook entertaining as well as useful . When a Web 2.0 site feels like "work" to someone, she will at worst stop using it entirely, or at best will minimize her use of it -- to just do the work she needs it to do. Either way, this will kill or at least dramatically limit the "network effect" of the site.
Most people, especially those trying to harness Web 2.0 for enterprise use, forget that "play" was highlighted by Tim O'Reilly as one of the essential elements of Web 2.0:
So just as the web itself is based of the strength of weak ties , social networking sites specifically and Web 2.0 generally are based on the productivity of fun ties . Lose too much of the fun or entertainment value of these sites by trying to make them too productive, and you'll kill the goose that lays the golden eggs.
I don't have time to go into it here and now, but I think all our talk about "consumerization of IT", "mass amateurization", and "democratizing innovation", we miss the unifying theme of all these trends: the advantages of blurring of the line between play (entertainment) and work.