Actually, quite a lot of religions start with multiple people being "in on the joke/scam", Scientology, Mormonism, Islam, etc.  You would be extremely hard pressed to figure out if & when Discordia got real followers, as its taken a very "post-modern" approach, but Discordia has had a very significant influence on Wicca, Neo-paganism, etc.  Its just not a black & whie issue, all it takes is for some people who really want to believe in a god to descide that Christianity sucks, and FSM is better in some post-modernist sense.