A chat window opened on my computer screen. A Facebook “friend” I didn’t know began communicating with me.

“Happy birthday, Tom!” wrote the woman.

“Who told you it was my birthday?” I wrote back.

“You did, Tom. You provided that information when you joined Facebook.”

“I did?”

“Sure, Tom. The information you willingly provide to Facebook – combined with sophisticated tools that monitor your online usage patterns – giv...