True, there's competition... but I find that what can be worse is running a forum itself.
I don't know what's with forums, but if you get any bad users, they can cause way more stress than Google possibly could.
I think it's because the trolls have a lot of time on their hands, and because forums have an obvious central power structure, they will do anything possible to shut YOU down should you step in to lock them out of your "kingdom". A diet forum is likely to have more than a few troublemakers who'll log in just to make fun of people and cause all sorts of other trouble.
Social networks on the other hand are moderated much more by the users themselves (users setting posts to private and/or marking bad users etc), so if someone starts getting ignored, often they'll blame other users more than the network owners. On a typical social network, you can basically keep someone around and anyone offended by them can easily shut them out. This helps keep the psychos at bay.
I'd say the niche social network thing is a safer these days, unless you're already used to running forums and have jedi abilities to admin them correctly.
Maybe check out BuddyPress (which has it's own minimalistic forum features) and Wordpress, SocialEngine, etc. The Wordpress/Buddypress or even Wordpress/Vanilla combo will still get you great SEO, but it's easier to moderate. Especially if you outsource some of the comments burden to Disqus.
