sock puppet (pejorative term)

In recent years, usage of this phrase as a pejorative has tended to refer to fake online accounts, however, originally it meant someone who was being manipulated by someone else as thoroughly as someone does when they put their hand into a sock and use it as a form of puppet. 

It's original use was, in part, a term of crudity - but the meaning of someone who is being thoroughly controlled or deeply is an agent of someone else (such as "enforcers of conformity", agents-provocateur, and other agents of the elite(s) ) still actually applies - fake online accounts are just one form of the original, wider term.