On Nov. 19, the FBI announced it had charged four men for plotting to join al-Qaeda and the Taliban and kill American targets.

Perhaps the defendants, who are all either U.S. citizens or lawful permanent residents, deserved to be arrested. However, the way the FBI gathered evidence against them should send chills down the spine of anyone who values free speech and other civil liberties.

To build their case, investigators relied heavily on content the four suspects posted, “liked,” commented on, and shared via their Facebook accounts.

The case begs the question: What is stopping the Feds from building a terrorism case against Occupy participants, anti-war activists, or others for sharing – or just “liking” – content that the government views as radical or extremist?