There are dozens of anime forums, gaiaonline.com having its fair share of otaku.
I think attention span has a lot to do with it. Forums have a certain popularity among high school students, who may spend quite a bit of time around computers with nothing to do during the day. I'll admit that I started my Gaia account when I was bored and in my Junior year, but now that I have a lot of things to do, I can only dedicate myself really to something that captures my interest.
When you're younger, you have more time for seven hundred webcomics, three forums, and a page alone dedicated to addictingames.com, but when you have two jobs, school, and a social life, you can only take breaks for one or two websites periodically through the day : D.
People get older and they put away "childish things". People graduate high school or college and get so busy with their outside lives that they get out of the habit of posting and then they forget the site existed, which is sort of what happened with me, especially after the sundering of the original website. I lost interest for a while and stopped logging in for two years. Then I got a random e-mail and decided to come back.
I like this forum specifically because it is so well moderated, and because we have a lot of old grumpy otaku *coughLenMiyatacough* that make life interesting around here while I should be doing Criminal Justice Classwork.