Social media in its various forms has a built-in problem. I see it as a social capital phenomenon that works very much like Gresham's Law. You know, the idea that there are two types of currency in circulation, the bad tends to drive out the good. This is why people tended to hoard goal (or even silver), and spend greenbacks.
In the case of social media it seems that over time various fora (Twitter, letters to the editor, blogs, web sites) tend to become increasingly contentious or even insulting. As a result, there is a slow dropout of the more pleasant contributors; and more acrimony as the number of feisty people increase in numbers and expressions. That seems to be also true with respect to blogs.
Let's face it: 90-95% of people post comments to share information or for enjoyment; when it stops with that payoff, then they're going to go away.
To date, I've had only a few to come on to Alabama Noise to start something. And those that do get some tolerance; but only so far. The old rule applies: Don't feed the trolls.
I can't think of a better solution for this dilemma. All I can do is describe it, and hope that some forms of regulation suffice. At least some form of moderation and throwing out the worst should suffice.