Trolls have taken over
The claim that trolls have taken over a wiki, newsgroup or mailing list or IRC channel is surprisingly common. For instance, according to Larry Sanger, its first chief editor, trolls have taken over Wikipedia and made it anti-credential, even anti-expertise.
According to its GodKing Jimmy Wales, however, it is necessarily anti-credential. Sanger being driven off by trolls was thus probably inevitable, as Sanger had credentials and expertise. Wales may believe this as a result of his own self-interest: Sanger being driven off let Wales claim credit for all innovations Sanger had made, and Wales has no credentials nor expertise that would actually let him run a genuine encyclopedia. So there is reason to consider Sanger's opinion carefully, not to dismiss it as Wales does.
Another explanation is that Wales and Sanger are both trolls, but other users are less so.
 Not the same as claiming "taken over by trolls"
Please note also that the claim that trolls have taken over is very different from the much more problematic "term:taken over by trolls". The term:have implies possession and therefore indicates a sort of property rights and the requirement to defer, e.g. to troll-friendly policies or to DuneCat. However, the term:by implies authors and/or commitment, both of which are inappropriate where trolls are concerned, since such a loosely defined entity can neither author nor commit. However, a loosely defined entity can and does often assert power, and may actually gain power from being so loosely defined.