Earlier quoted context omitted.
My daughter at 12 got sucked into Tumblr and developed an eating disorder. I'm really sorry to hear this, having gone through a similar thing with a now ex-significant other. She too became very active in the segments of tumblr you identified and others (showed me many of them, some seemed like teenage angst in blog form, others were far more...worrisome), developed an eating disorder along with severe anxiety issues…
These online groups have the same advantage that many other cults have over their victims: they can re-try until they find enough buttons to press that work to bring out the vulnerable people. It's like negotiating against a much more experienced adversary: you're going to lose. The only way to deal with that stuff is not to get exposed to it in the first place. Something similar happens with advertising, there too t…
It seems like understanding how these systems work would be a prerequisite for protecting your loved ones (or even yourself!).