Earlier quoted context omitted.
As a white middle-aged male, Tumblr is the only place I've experienced what it is like to be on the receiving end of racism. It's quite fascinating to see just how vile some of it gets. Of course it is only fascinating to me because I can trivially withdraw from it and not be particularly affected.
My point about Tumblr is that it's a huge site containing a lot of blogs. Seems like half (or over half) is porn. Most of the other half seems dedicated to rabid fanbases of various bits of pop culture. There's some politics like everywhere else on the net, but I have no idea why so many people here, including my downvoters, think it's this despicable bastion of frothy far leftism when that's only a small part of it.…
The combined result is that, on Tumblr, you'll subscribe to people for one thing, and then inevitably they'll "signal-boost" other things into your feed that you didn't sign up for. The most often "signal-boosted" thing is Social Justice discourse. This is why people get the impression Tumblr is a Social Justice website: if you subscribe to blogs about porn, cats, and tech, you'll end up with a dashboard containing porn, cats, tech, and "signal-boosted" Social Justice.
Personally, my problem isn't with Social Justice, but with "signal-boosting" itself. I (very carefully) use Tumblr, and enjoy doing so, but only by strictly following a policy of unsubscribing from any blog that "signal-boosts" anything. I think this means that I end up following only blogs that are completely disconnected from Tumblr's social graph, though, so I don't know how precisely I can be said to be "using Tumblr" at this point, rather than just being subscribed to some blogs that happen to be hosted there.