Moving to challenge Google and other emerging location-based services, Facebook on Wednesday introduced Facebook Places, a way for users to expose their location to other Facebook users and applications and to locate their friends.

Places allows users to share their location in real-time using an iPhone or a Web page accessed through a mobile browser that supports HTML 5 and geolocation.

By accessing Places through Facebook's iPhone app or the touch.facebook.com site and pressing the "Check In" button, the user sees a list of nearby places. He or she can select an existing place, search for other nearby places, or add a new one.

Checking in sends a message to friends' News Feeds and shows up in the Recent Activity list associated with the relevant Place page.

Places also allows users to tag friends who are present in a location, just as they can tag friends in photos posted to Facebook.

The service is currently available only in the U.S. but Facebook is working to make it more widely available.

Facebook is exposing Places data to developers through its Graph API. It anticipates developers will employ Facebook users' location data to enhance the social experience of various mobile and Web applications.

The company also anticipates criticism from privacy advocates, as can be seen in its insistence that there's no privacy risk associated with Places. "It's important to remember that all access to location information through the Graph API respects a user's privacy controls, which can be viewed and changed on Facebook.com or m.facebook.com," explained Facebook Places engineer Ben Gertzfield in a blog post for developers.

Facebook product manager Michael Sharon offers the same reassurance in a general interest blog post. "With Places, you are in control of what you share and the people you share with," he wrote.

The ACLU of Northern California, however, isn't buying it and claims that Facebook has failed to include important privacy safeguards.

The civil liberties group argues that Facebook makes it easy to agree to allow friends to tag you but doesn't provide an easy enough way to opt-out. It also argues that users don't have enough control over who can see their location in the "Here Now" list, which is turned on by default.

As it works with partners including Gowalla, foursquare, Yelp and Booyah, Facebook Places appears to be destined to compete with compete with Google Places, the search company's location service for businesses.

Google last month introduced its Places API to allow developers to add information about local businesses and points of interest to map-related applications.

Gartner analyst Ray Valdes characterizes Facebook Places as a necessary defensive move to address the growing popularity of location services like foursquare and Gowalla. "It was actually a pleasant surprise that Facebook didn't try to stomp out this ecosystem," he said in a phone interview. "Instead, it gave them some wiggle room and an opportunity to innovate to stay ahead of the juggernaut."

In time, Valdes expects Facebook's focus to shift from user engagement to driving revenue through Places, at which point competition with Google Places could become more pronounced.

"Google has been extending the scope of its coverage from the Web to the real world, along with others," he observed. "They're moving not only from Web content to the social Web but to the Web of things and places in the real world. Facebook Places has more of a social dimension while Google Places is more focused on the information dimension. But eventually the two services will converge and become directly competitive."