Facebook is planning to track how many times a week you go to the grocery store, and every other store, and it will share that information with advertisers.
Using the location services on your phone, Facebook will keep a tally of who goes to what stores, and show the anonymized numbers to advertisers, as evidence that buying ads on Facebook is getting people to visit brick-and-mortar businesses.
It's a great thing for Facebook, which will now have excellent data to prove (or disprove) on a user-to-user basis what a store is getting for its advertising dollar. But it's a pretty frightening idea that a company will have information not unlike your credit card statement all from location services data.
Of course, turning off location services is fairly easy on most devices: something to think about.