A simple business model that would be coherent with your current users would be let them mention what they do to loose weight, identify commercial elements (not just weight-loss powder, but also say, iTunes apps, or a organic walnut producer) to sell mentions, or better coupons to groups that pledge to follow a similar regimen. That way, you can have not only friends, but stranger who use the same technique to help you out.

Sponsoring would be a good idea if you fail to identify anything relevant automatically: let the users name a product that they use to loose weight, and associate their gains to it.

I'm not sure how competitive you want this too be (hopefully, not too much) but you might also encourage treats to your members. Let them tell they friends (on Facebook/twitter) about it, and their friends pledge a present if they reach certain goals and get a recommendation fee from those merchants. Maybe consider a two- or three-sided gift not too punish and burden too much those who failed: a trip to Paris below a certain threshold, a wine & walk tour of California below a higher threshold, and a sweat-your pants off in a Cheyenne lodge otherwise.