There was an interesting essay I read (you can find if you browse wtfsocialjustice long enough) written by someone with an actual disorder explaining why these multiple/headmate/otherkin communities on tumblr can be really dangerous to people with actual mental problems. Basically, they (poes, speshul teens) surround these people (actually mentally ill, especially newcomers) with positive reinforcement and validate their delusions. These communities will insist that there's nothing wrong with the person, that it's everyone else with the problem. The OP said that this can convince an ill person that their delusions are reality and, due to being constantly surrounded by people reinforcing that idea, may never experience the moment of clarity needed for them to realize that they may need to seek help and treatment. The OP said they started treatment before tumblr was ever a thing, but they're convinced that, had they been not diagnosed and not medicated at the time these tumblr communities became huge, they never would have sought the help they needed.