The members of MBTI forums are a population that's quite strongly skewed in the direction of N's (first and foremost) and introverts (secondarily), as
dramatically illustrated by the PerC and Typology Central stats in the spoiler in
this post.
And I'd say that's for the same reason that Jung and Briggs and Myers and Quenk and Berens and Nardi are all INs — because INs have a substantially greater tendency to be interested in personality types than other types.
Jung felt strongly alienated from the cultural mainstream, and it sounds to me like most of the predecessor typologists whose theories Jung reviewed in
Psychological Types were fellow INs who I suspect were also, like Jung, partly moved to formulate their "different types" theories by the fact that — like a sizeable percentage of the INs in (I assume) most eras — they felt significantly alienated from the majority of their fellow men.
And if INs are the types most likely to feel alienated from "normal society," you'd expect that a not insubstantial number of INs who visit MBTI forums would be pleased to find themselves among a higher percentage of
kindred spirits than they tend to find out in the so-called
real world.
That has nothing to do with whether MBTI forums are "a joke," of course. On that issue, I'd just say that some posters are more joketastic than others.