A shill is an agent who tries to commit espionage against the public as part of their job.
Paid to do so, their methods vary, classically by pretending to be a member of the public an causing social disruption to ineffectualize the infiltrated group.
What distinguishes them from other agents of espionage is that they try to work against the best interests of the public.
Like evil spys but with much less credibility an aptitude.
Shills are worth less to society than an average puddle of shit, an Facebook's got most of them..