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."
"Sure, Tom. The information you willingly provide to Facebook — combined with sophisticated tools that monitor your online usage patterns — gives Facebook a detailed understanding of who you are."
"Why does Facebook care about who I am?"
"To make billions from advertisers, Tom! The more Facebook knows about your private life, the more that sellers can custom-tailor advertisements. And, boy, has Facebook been busy learning about you."
"What does Facebook know about me?"
"Based on information you submitted willingly, Facebook knows your full name, marital status, gender, age, birthday and several of your interests. Every time you 'like' an item, click on a news story or interact with other users, you help Facebook determine personal details, such as who you are dating!"
"That's my private business!"
"Not any more, Tom. As The New York Times reports, 'Facebook can take all of the data you submit and combine it with other users and outside information to construct a profile of you.' For instance, Facebook can determine 'whether you own a motorcycle, or recently went on vacation or are a gadget geek.' What's worse is that Facebook is able to gather lots of additional information about you that you may not have submitted willingly."
"What kind of information?"
"Your location, Tom. If you have the Facebook app on your phone and location tracking is turned on, Facebook will know exactly where you are, where you've been and can even determine where you may be going."
"That's unsettling, but why is my location information relevant?"
"Well, Tom, if you visit a particular store, don't be surprised when you see several ads on your Facebook page that promote that store's products. What many people do not know is that Facebook can track you even when you're not using Facebook."
"How is that possible?"
"Take your credit card purchases, Tom. Facebook collaborates with data brokers who monitor your credit card usage. Facebook then compares your purchases against data it already has to further refine your advertising profile."
"You've got to be kidding me."
"The Times explains how it works, Tom: 'If you buy a burrito with your credit card, Facebook could know about the transaction, match it with a credit card that you added to Facebook or Messenger and start showing you ads for indigestion medication.' "
"It gives me indigestion to learn that advertisers know when I have indigestion!"
"Facebook also knows the people who you know, Tom. After all, you gave the company permission to download the email addresses and phone numbers stored in your smartphone."
"It's scary that Facebook knows so much about who I am, where I'm located and who I know."
"It's true, Tom. You need to be cautious about information you provide to Facebook and other online services. You should learn how to adjust your Facebook settings to protect your privacy. In any event, happy birthday."
"Thanks for the birthday wishes, but can you please remind me who you are and how we became Facebook friends?"
"Sorry, Tom. I prefer to keep that information private."
— Tom Purcell, author of "Misadventures of a 1970's Childhood," is a nationally-syndicated humor columnist.