I've just read right through this discussion, and so rather than add to the inbox clutter, I'll try to summarise all my thoughts, and apologies if I have to say "someone said..."
I agree with many of the points made; finding active networks is hard; blogs can get few comments - I feel that in theory, for every post I make, I ought to do at least one comment - in order to try to redress the imbalance. Unfortunately, I don't have time to comment on every post that I read. And, I assume my audience has the same view.
Others have mentioned "Ning", and the need to login to 3,000 different communities there. That's something I also agree with & as such find Ning hard to recommend to students - as it makes it hard for them to see their overall contributions to different communities, in the way that they can with, say Facebook. That said, I don't particularly like facebook, but I feel that it is useful, as others are there. It's almost akin to the days when I used to go into the smoking room at break times when I was teaching in schools. Not because I smoked, but my friends did & if I wanted to talk to them, I had to go there.
Like others, I'm neither evangelist nor sceptic. I think that Social media has a lot to offer, BUT, it's not for everyone, nor for every event. The problem that we have with using it is that even for the keen users, not all like the same platform; and, currently, many social media tools are rather blackhole-ish. You have to be in them to benefit from the information, and if a particular site doesn't suit anyone for whatever reason, then you have a problem. Roll on "proper" OpenSocial