Why do trolls do it?
I believe that most trolls are sad people, living their lonely lives vicariously through those they see as strong and successful.
Disrupting a stable newsgroup gives the illusion of power, just as for a few, stalking a strong person allows them to think they are strong, too.
For trolls, any response is 'recognition'; they are unable to distinguish between irritation and admiration; their ego grows directly in proportion to the response, regardless of the form or content of that response.
Trolls, rather surprisingly, dispute this, claiming that it's a game or joke; this merely confirms the diagnosis; how sad do you have to be to find such mind-numbingly trivial time wasting to be funny?
Remember that trolls are cowards; they'll usually post just enough to get an argument going, then sit back and count the responses (Yes, that's what they do!).
Where are you likely to find trolls?
Wherever they are tolerated - this means forums and networking sites that have loose or lazy moderation, and places like Usenet (newsgroups) where there may be no moderation at all.
Where trolls are successful is a slightly different issue - some forums allow members to set an 'ignore' function, so they just do not see troll posts, after the first one, and some specialist forums are tightly focused so that troll posts stick out a mile and are ignored - it's larger, more general forums that have issues, as it only takes a couple of members to be drawn in for the whole forum to suffer. They also thrive on 'opinion' sites, such as politics and religion, where feelings often run high anyway. But, again, only if moderation is weak.
Usenet has a special problem, as moderation is impossible; for trolls to be controlled, requires committed members using their killfiles effectively, and avoiding accidental cross-posting - which is all too easy with many newsreading software setups.
Some career trolls have set up their [own] forums to exchange notes. These rarely thrive, as most trolls are not team players, and they simply fight among themselves. in other cases, they become centers for cyberbullying (see
stalking); while that's a sad development, it makes it easier to have the sites closed.