Earlier quoted context omitted.
The medium is the message. Twitter is designed like a platform for harassment, so a lot of the content on Twitter is harassing. Tumblr is designed like a platform for radicalization, so a lot of the content on Tumblr is radicalized. It turns out that your very advice ("don't make your social circle out of the people who [create weird and distateful fringe content]") is the exact same echo-chamber-and-radicalization-m…
But Twitter makes it so that's all up in your face. I haven't used Tumblr hardly at all, but it seems like it's optional to participate in that, since things are a little more partitioned. Anyway, I'm interested in hearing more about the radicalization thing, which I haven't actually observed first-hand, but several people in the comments here have mentioned. What interaction elements do you think lead to it?
It's not entirely optional to interact with radicalised elements on Tumblr. The tagging system lets them find you, and then they can brigade you if you fall foul of whatever pet cause they have.
This happened to me when I'd posted a photo of my dog. She's my pride and joy, and it was a harmless and cute photo of her jumping to catch a ball in the air. My mistake was using the caption "Good girl!" and tag #goodgirl. (I also tagged #dog, #fetch, #springerspaniel, #spaniel.)
I was flooded with follows from people, mostly men but also some young-ish girls, in the "daddy dom" community (their main tag seems to be #ddlg, but #goodgirl was enough for them to find me). That was a pretty disturbing subculture with definitely uncomfortable elements of abuse built right into the foundation of it.
I was also reblogged and targeted by a group of people who had decided that by "gendering" my female dog, I was a bigot. They mercilessly harrassed me. They flooded my inbox with abusive message: "dirty cis scum, kill yourself", things like that. They reblogged my photo saying some pretty vile things about me and my poor dog. In their reblogs they utterly misrepresented what I'd said, reposted it endlessly, called me transphobic, a bigot, threatened to dox me.
In the end I abandoned my entire blog there and deleted it. As far as I know, there's no way to get my dog's photo removed from Tumblr or get the hateful reblogs taken down. I reported them all for abuse and harassment, but Tumblr did nothing. Tumblr's staff don't care. They would never respond on Twitter or by email or to my reports through the form on the site. Just radio silence.
This is purely anecdotal, I know. I know someone else who also accidentally drew the ire of Tumblr and ended up having to delete their blog and leave the platform to escape a sustained campaign of harassment and threats from a similar group of people. Again, Tumblr-the-company did nothing to stop it.