Define "online social"; if you're trying to emulate Facebook, MySpace or whatever, but with a fitness theme then you've already got models to follow there (page personalization, photo albums, friends lists, private messages and a public board/wall per profile are all good starting points) but at that point you're competing with established social networks.
If what you're trying to do is build a community for your forums, well that's a very difficult question to answer... you really have to have a strong, active userbase and trying to "create" that just doesn't happen... you've got to offer a service or benefit that people can't get anywhere else, or is at least superior. Communities like SomethingAwful, Fark, etc. have been building themselves up for a long time and LowTax at SA has even publicly stated that it mainly happens by accident, it's not something you can buy.
I guess the best thing you can do is just personally stay active on your site and be visible; it does make a statement to your user community if they see the admins participating and hope that other people follow your example. Things like contests and polls to get people directly involved can also build some temporary momentum; just have to capitalize on it and ride that wave.
