Surely, you haven't seen the hilarity of The Oatmeal's comment section (the author is notorious for epic counter-trolls.) Also, one of the most fun parts of Yaoi 911 and Teahouse (both wildly successful yaoi comics) are their comments section campers and the often dramatic speculations from the fans. In these sites, the comments sections are often as entertaining as the webcomics, or even more! But, I suppose this all depends on the audience.
As for Facebook, I actually deleted my accounts for all of my webcomics because the drama and negativity (usually because my comics offended people who didn't seem to understand what NSFW meant.) The trolling there by people who more often than not only read a single page was terrible. Email, Twitter and G+, forums tend to be civilized.
The comments on my webcomics site tend to be overwhelmingly positive or politely constructive/informative and I enjoy them very much, even if they are few and far between. I consider them treasures and validation that I am doing something really right. With the anti-spam tools available now for WordPress, killing overly negative comments is a snap.
Visit Catnose Comics
and Velvet Rasputin
to see my comic art and graphic novels.