In some communities it is possible for a smallish group of well-motivated, well-organised, knowledgeable people to take control of the community resources. This can be an important tool, as in the case of CommunityMembers solving a ConstitutionalCrisis, or it can be a threat, as in the case of attempts by attackers to wrest control from the original community and force it to StartAgain elsewhere.
Where a community is controlled by a company of some sort the easiest MilitaryCoup may be to simply takeover that company by buying enough shares.
One of the motivations for the RightToFork.
The "Meow Wars" were essentially a takeover of one usenet group by the occupants of another - see http://www.xahlee.org/Netiquette_dir/_/meow_wars.txt. Here the tools used are flooding and flaming to make life intolerable for the original occupants.
IRC suffered from coups, where one group or another tries to gain exclusive access to operator privileges, taking advantage of server splits, or using social engineering to gain a foothold.