Sockpuppet

About

A sockpuppet (also sock puppet, sock-puppet) is a user account on a social network that is created for the purpose of providing credibility to a pre-existing account by creating the appearance of collective agreement. It is a small-scale version of astroturfing.

User A (the sockmaster) creates user accounts B, C, etc. under different names. (A competent sockmaster will create an avatar, fill in substantial background details, and establish a posting history as well, in order to make these accounts seem more like real people, since experienced users will often block a bluehead at the first sign of trouble.)
User A makes a controversial statement which would not otherwise be taken seriously.
User A then logs on as User B and makes a comment agreeing with the first statement. (A competent sockmaster may wait until someone disagrees with the original comment, in order to make the flow seem more natural.)
Depending on how many sockpuppet accounts the user has created, how much time/energy they want to put into the project, and how the marks respond to the initial provocation, the sockmaster may then repeat the process with accounts C, D, etc.