I recently came across a website that is similar to Mixi but geared toward language learning. I liked it so much that I helped translate their user interface into English (my updated interface isn't in place yet, but it will be in about a week or so).
The difference between Mixi and Lang-8 is that the later is geared toward language learning and has some pretty cool sentence correction functions. It looks very promising, and unlike Mixi, it is open to the planet.