A member of a crowd with no originality or voice of his own. One who speaks or repeats only what he has been told. A tool. A cretin.
"Ya think he'll do it?"
"He's a lemming, he'll do anything he's told."
A lemming refers to a purchase/wished-for-item which results from reading an enthusiastic post about a new fabulous product. Overcome by compulsion, readers follow like lemmings diving off a cliff.