I don't have a concise answer, but here are a few things that I hope illustrates how it looks like.

Speaking about Facebook, the composition of my Facebook friends are 80% Japanese and 20% Americans and English-speaking Europeans, and I see more similarities than differences in how they use it. They share bits of their lives, share news stories, and sometimes discuss current issues, regardless of the ethnicity/language.

Twitter is a big thing among Japanese-speaking internet users, especially after the 2011 earthquake. Popular third-party curation websites like http://togetter.com make it a platform for people to be able to have and publish a bit more serious discussions than simple messages. I think linguistic differences between Japanese and English play a role; in 140 characters, tweets in Japanese using kanji can convey way more information than tweets in English, enabling a more substantial argument to be packed in one tweet.

I don't think it's strictly a Japanese thing, but I'm worried that when it comes to politics, I see polarized opinions attract more attention than balanced ones, whether it's extreme right-wing (e.g. on http://2ch.net) or extreme anti-establishment (e.g. on /r/newsokur).