It's not just an internet thing; since Sybil, multiple personalities became an identity to aspire to. What the internet adds to it is that all of these individuals network together and start setting up a common lexicon, demands that the condition be treated as what they say it is, and a bunch of rules dividing "authentic" MPD from fakers. At some point an industry arises to prey on them.

It's like fandom, basically, except with medical treatment and the sense of being an oppressed, abused minority.