In Internet lingo, a "sock puppet" is a false second identity used for deception. It originally meant a fictitious identity created by a user to praise or defend his "real" one (e.g. "I've been reading this guy's posts, and I think he's got a valid point"). These days we also use "sock puppet" to refer to someone who creates a second identity to evade bans, stuff polls, or to caricature some opinion or position and make it look ridiculous.
The FFF's got a long history of sock puppets, especially of that last definition. As long as they're meant for humour rather than deception and are a poorly kept secret, they've been tolerated.