Consolidation, my man. Like others have said, Reddit and 4chan act as "metaforums" where each subsection is it's own website by itself. Facebook and Twitter have compounded this. Also, a lot of people like ego boosting for what they've said, or at least confirmation in their beliefs from others. Reddit has upvotes, you can be "top commenter" on Youtube videos, and some people treat Facebook "likes" like black tar heroin. Traditional forums can feel like yelling into the ocean, you always wonder if anyone is even going to read that post that you put time into unless you quote someone or try and make it happen.
Also, there's the issue of smartphones and tablets. Every damned thing, even traditional websites, has an app to make mobile access more efficient. For an example, I tried to get someone I know to join the Escapist because he loves video games and he isn't a dick (seems to be the type). His first question was whether there was an Escapist app so that it would be easier on his iphone. I said no (I don't think there's one), and he relented and went to visit the website. That's when I had to break the news that you had to pay money in order for the website itself to be formatted properly for mobile devices. It's actually kind of hard to convince people to join us, and it made me wonder why it is I come here every day. I guess it's good to get "the pulse on what gamers are doing/feeling at the moment" or some such thing, because it doesn't seem like a social thing at all.