Personally, I haven't had problems with Facebook or young people on Facebook and I'm in numerous groups (car ones, generally related to BLARG). For the group that I help manage, in the last 2 years we've had two significant issues to sort out, neither involving people under the age of 30 (although for one, the risk was to younger members of the group). One was relatively easy to deal with as the Police has some involvement making it quicker and easier to address, The other one was a bit more of a challenge as it involved potential liable. However, after about four hours work it was sorted to everyone's semi satisfaction, which was the best that we could hope for.
My understanding (I think from research covered on the news) is that young people are starting to move away from Facebook to other social networking sites and one reason given was that their parents are often on Facebook too, making it less appealing.
I think the AROnline Facebook group is a good example where it does work well and is generally friendly with good banter. In that respect, personally I do find Facebook friendlier than a forum set up. Keith Adams seems to have got both the AROnline site and Facebook group working well together, but the AROnline site is a different type of site to TMF, so they're not really comparable.
I also like that fact the generally people use their own names on Facebook (I know not all, and I know you can't tell if it's a pseudonym) and I do prefer real names to forum names - that does feel more friendly to me. Quite often you can see what they look like and know a bit more about them than you can with forum members. It is rare for someone from a forum who I've never met before to come up to me at a show (and vice versa), but I'm regularly greeted at shows by people introducing themselves as 'so and so from Facebook' as they've recognised me.