This is kinda funny, but if you choose to enter profile information on mixi and then make it absolutely private (even from your friends), you can still extract it.
Go and download this bookmarklet. Save it to the bookmark bar on your browser.
Go to the page of a friend on mixi and click it. Hilarity ensues.
As far as security leaks go, it isn’t so bad since you have to friend the person before it shows up. However, I doubt that anyone is actually aware of that fact.
I love meeting people on mixi (since it is the rare avenue of socialization), but stuff like this is going to make everyone get up and leave (and that’ll suck).
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For those curious about what it looks like:

-edit-
I’ve been doing a little more research into this and it doesn’t look as sinister as it did originally. Some of this information is hidden from the “friendship” profile screen because it is not interesting (?) after someone has already friended the person in question. It appears to be a misplaced usability optimization rather than anything else.