Oh, sorry! I've now looked for some information on Google, but it seems people are rather blogging instead of using forum...?
Have you heard of "2ch"? It's a huge community and people discuss about quite much anything. One huge problems of 2ch is that people don't really have "common sense" sometimes and conversations often contain disgusting postings like libels and abuse.
I don't really recommned 2ch, but you might want to give a look...?
I know; I was surprise too. But to be honest, when I googled it, I even didn't know what to type in Japanese if I want to type "forum".
That's how less I don't know? Or, that's how less common the forums are among Japanese...?
When I wrote "blog", I meant more personal bloggings, not social networks.
It of course could be, like facebook and/or Twitter, but there's "ameba" blog (ameblo), for example. I believe people prefer writing blogs and talk about any topic with people who come and visit their blogs.
If you write "アニメ ブログ" to google, you'll find quite a many lists of blogs about anime/manga.