The new social networking feature fuels new worries about privacy.

Where are you now? If you’re a Facebook user, that question became a little easier for the 499,999 million other users to answer last week—and kicked off yet another set of concerns among privacy advocates.

Facebook Places is a location-based social feature that, like Foursquare and Gowalla, lets users check in at places and advertise their location to friends. Like the other networks, Places users can check in via iPhone and send their location to Facebook from, say, a restaurant, club, or museum and see other people who have checked in at the same place and have agreed to advertise their location.

The problem, according to privacy advocates, is that you can also “tag” a friend and post his or her location to Facebook— even if your friend doesn’t have an iPhone and hasn’t necessarily opted in to the service. Close on the heels of the launch, the ACLU released a statement expressing its concerns:

“Places allows your friends to tag you when they check in somewhere, and Facebook makes it very easy to say ‘yes’ to allowing your friends to check in for you,” read the statement, released late Wednesday night. “But when it comes to opting out of that feature, you are only given a ‘not now’ option. ‘No’ isn’t one of the easy options.”

The organization also warned that Facebook had already opened up location data to third party sites: “This means that your friends’ apps may be able to access information about your most recent check-in by default as soon as you start using Places.” It also provided a Places resources page for managing your privacy settings.

And if Facebook CEO Mark Zuckerberg emphasized in a press conference for the launch that “The main thing we are doing is allowing our users to share where they are in a really nice and social way,” some are more focused on the potential that location-based social networks have for doing harm.

In this article for the U.K. newspaper the Guardian, for instance, writer Leo Hickman used Foursquare to “cyberstalk” a public relations executive and expose the dangers of opening yourself up to others who would be doing the same thing for more nefarious purposes. Having located her on Foursquare and cross-referenced her on Google and Twitter, Hickman says, “Louise is about to meet her new digital stalker.”

While the new Facebook feature, according to this New York Times article, is anticipated to help the company compete with Google, other location-based services such as Foursquare, Gowalla, and Yelp so far see Places as yet another distribution channel.

What do you think of Facebook’s new Places feature and other location-based social networks? Are they fun or unnerving? And is it responsible to tag someone’s location without their consent?