If the Free! characters had TumblrsHaruka: water and swimming and the like (he once followed a blog with "watersports" in the url and was very surprised to find out it wasn't about swimming).
Makoto: pictures of cats, nature, cats, humor, anime, other cute animals, cats, DID I MENTION CATS?
Rin: sports, fitness and a lot of black and white/negative and hipster stuff.
Nagisa: humor, horror anime, food porn and a shit ton of selfies. He sends Rei a lot of stupid anons, too.
Rei: anything he considers "beautiful" at the moment. He reblogs all the "science side of tumblr explain this" actually writing an essay explaining the thing because he'd totally do that.
Gou: muscles galore, but also fashion, anime and some yaoi.
Sousuke: he only made a blog to stalk Rin and send anon hate to Haru.
Momo: humor and poRN.
Nitori: he's actually tumblr famous but nobody knows.