If you are an English teacher and looking for Japanese students, you might want to try Globee.com. It was launched last month and counts nearly 4,000 members, most of which are Japanese wanting to practice their English. Pimp yourself there and you're sure to find students easily. It's free, and no need for invite.
Might not be the most interesting hangout for hardcore FG's, but if you always wanted to use Mixi and couldn't read the kanji, or simply you want to meet new people, here's a chance to find Japanese who are interested in interacting with gaijins in the first place...
(Disclaimer for those who don't know: I do work for Globee)