Facebook is increasingly being used in courts to decide who is—and who isn't—suitable to serve on a jury, the latest way in which the social-networking site is altering the U.S. court system.

Prosecution and defense lawyers are scouring the site for personal details about members of the jury pool that could signal which side they might sympathize with during a trial. They consider what potential jurors watch on television, their interests and hobbies, and how religious they are.

Josh...