Using Mixi To Learn Japanese
Mixi is probably the most similar to Livejournal, in that it is centered around journal entries, and not so much “pokes" or photos or the like (as in Facebook). This means that Mixi will excel at having you work with actually writing Japanese, one of the most difficult aspects of the language to master.
In addition, because the entries are more bloggish and casual than your typical dry essay that you’d be writing for a formal class, the time will pass more quickly, and learning Japanese won’t seem like quite as much of a pain.
Reading plenty of Japanese on-site is the other bonus to your language skills. You’ll be reading other people’s journal entries as well as comments on your own, immersing yourself both socially and linguistically. People tend to write in short, simple sentences, often from their phone, so it’s great practice.
However, jumping into Mixi as an absolute beginner to Japanese is probably not the best idea. Having at least intermediate skills in reading and writing Japanese is a good prerequisite before you can begin really making the most out of Mixi as a fun social environment in which to practice your skills.