Facebook’s new Nearby Friends feature helps you track … well, your friends.

Activate it. Find out who’s nearby. Or let friends know you’re in the area. Meet for a drink. Take in a movie. Use Nearby Friends when traveling to another state. Friends you haven’t seen in years can now know you’re near. Cool.

Facebook announced the feature today in a post written by the Nearby Friends lead designer Andrea Vaccari. The feature is optional. It will run on both Android and iOS devices and will go mainstream on Facebook very soon. VentureBeat was not given the chance to chat with Vaccari directly.

According to Vaccari’s blog post:

“Sharing your location with Nearby Friends goes two ways — you and your friends both have to turn on Nearby Friends and choose to share with each other to see when you’re nearby. Your friends will only be able to see that you’re nearby if you share this info with them and vice versa.

If you turn on Nearby Friends, you can also choose to share a precise location with the particular friends you choose for a set period of time, such as the next hour. When you share your precise location, the friend you choose will see exactly where you are on a map, which helps you find each other. Then you can meet up and spend time together.”

Opting in to Nearby Friends means you will occasionally receive a ping when someone on your friends list is within your vicinity. Or a group of them. Also, users may opt out, which means you won’t be visible to friends using it. This is important if you don’t like being tracked.

Google Latitude and Foursquare already offer similar features.Nearby Friends 2According to Vaccari:

“When Nearby Friends is on, you can see when your friends are traveling if they’re also using this feature and sharing with you. You’ll be able to see the city or neighborhood they are in, including on their profile. When you see a friend visiting a place you’ve been, it’s the perfect opportunity to send a recommendation for a great restaurant. You can also make last-minute plans to meet up with a friend who happens to be in the same place you’re headed to.”

This also represents a potential mobile advertising windfall for Facebook. It means the social networking site can know, in real-time, where you’re hanging out specifically — whether it’s at Billy’s Coffee, Daren’s Skateboards, or even McDonald’s. Targeted ads, the kinds that brands want to place strategically, is now yet another option. A Facebook spokesperson later told VentureBeat that it will not use Nearby Friends data to target ads. Nearby Friends 3