I'm not familiar with Korea, but I don't think there's anything particularly weird about Japanese social media. The obsession with privacy is eroding over time, but still relatively strong. Mixi basically died and was replaced with a combination of LINE, Twitter, Instagram, and Facebook.

Facebook is popular in some circles, but not nearly as active as in the US. It's almost closer to LinkedIn, like your professional face (even if not strictly about business like LinkedIn).

People often have multiple Twitter accounts for different purposes and it's incredibly popular. Maybe the only country where Twitter's still growing?

Instagram use is basically the same as the US I'd say. It's helped erode the privacy obsession, with more people posting selfies and such.

And LINE is the juggernaut being the primary social graph of Japan. It's the default for one-to-one and group chats, and some people use the other features like the wall-like feature, shared albums, etc.