Facebook's upcoming Nearby Friends feature can tell your friends exactly where you are.
Facebook is starting to roll out a new feature that broadcasts your exact location using your mobile device's GPS. "Nearby Friends" lets you know when your friends are close, and gives you the option to send them a real-time map of exactly where you are. It carries some decidedly Violation-Of-Privacy-flavored risks, so it's opt-in for now. But Facebook would really love for you to enable it and use it all the time, and maybe enjoy some location-based ads while you're at it.
People tend to get nervous when their physical location data is involved, but the GPS feature itself seems pretty innocuous. The "precise location" broadcast only lasts as long as you allow it to, and only gets sent to the people you choose. However, your general location seems to be available to all your friends, and you'll be notified if one of your Facebook friends happens to be near you (if you both have the feature enabled). Even if you don't know exactly where they are, a pop-up announcing "John Smith is nearby" is a little more information than I really want to know at any given moment.
Opting into the feature also turns on Location History for your account, letting Facebook know where you've been and serving you relevant advertisements. If you aren't comfortable sharing your whereabouts with faceless corporations and casual acquaintances, you can toggle it on or off at any time in your account settings.