Ok so this one (that is an affiliate link by the way) has a few people quite hot under the collar. Heres the thing though i am not certain it is a scam! The way i see it is these guys would make far more if it's gen than a scam but there's quite a bit of theory behind that. The plan itself is genius, i even marked it as it could easily take over facebook (man that didn't go down well) and very rapidly if genuine.
Of course there is no way of enforcing that you would ever get your shares, but that would lead to the demise of the site pretty quick... and why do that if you are onto a good thing?? Further, what is there really to lose ??