Facebook "is a big, big thing that the Israelis use", says Ehab al-Hussein, a spokesman for the Hamas-run interior ministry.
"Many people don't have security sense. They go on the internet and talk about all their personal problems such as with their wives or girlfriends," he says.
Israel's intelligence services can then contact people by telephone, e-mail or using existing Israeli agents in Gaza, and use the information to pressure people to become spies.