Like Alvis and others said, the drop-off in traffic* is not due to any bannings. It's the ease of posting on FB. I can upload pics directly from my phone. No third party photoboucket style BS. With phone cameras becoming so good now, I can take a better pic with my phone than with my real camera (I know you DSLR types will not think the same way. Just settle down, I'm talking about myself, only), and with one click its online. With the rise of FB Groups, and the ability to form one's own blog-style pages which do give you SOME of the archival capability of a forum, it seems forums have to offer something that FB can't. It isn't going to be ease of communication, or sense of community, as FB's instant communications do that better.
What do forums have? There is the archival quality: provided that photo hosting sites don't ruin that with asinine sharing policies.
What about legacy? A place like ARC (or MM) has been around for a really long time. There are people here whom I have been following since 2005. I've watched them improve and evolve as modelers and as people. I can look back on my own posts and cringe at some of the dumb things I said as a 20-something in 2005, or look at my old work and realize that even though I'm not a great modeler, I'm better than I used to be. I've built friendships here. I've witnessed old friends have kids, and have even learned of the deaths of others. This once again relates back to the forum's archival quality. Maybe this is the place for the old timers to go? I don't know. That could be an angle to play up.
As for abrasive types, and heated threads: I think they actually increase traffic. As one of you pointed out above, this has been one of the most active ARC posts, in a long while. For every dumb comment made in a heated thread, there are 10 times as many that are thoughtful. And it is thoughtful conversation that attracts traffic. So maybe the mods should let arguments run their course. If somebody is being an absolute jerk, spewing hate and not counter-argument - then by all means ban them, but let the threads run. They're informative, and much of the time, entertaining. Most of us here are adults, and can handle it.
And the kids - well it's not like they don't know that kind of talk as it is.
*I don't think the drop-off is universal to all forums. Places like BM are still very active. Maybe the Brits have better retained the attention span necessary for forum posts.