The program for mobile phones will lets its users "share where you are with your friends, see where your friends are and discover new places around you," said Mark Zuckerberg, Facebook's CEO at a press conference.

The addition of "Places," first for Apple's iPhone (with an app available Wednesday night), may have a huge impact on Foursquare, the leading location-sharing site, which Facebook tried to buy earlier this year.

But some say the bigger impact will be on Google. Evidence of that was clear Wednesday, with representatives from Foursquare, Gowalla and other location-sharing based sites at the press conference, giving Facebook huge props for starting the service. A Google rep was not present.

Mobile location-sharing programs — telling friends where you are when you're out at a bar or restaurant, for example — has seen huge growth in the past year. (Places for Android-based smart phones and for BlackBerrys is not yet out, although users can access the app on their phone's Web browser, Facebook said.)

Foursquare has been registering more than 1 million "check-ins" a week since February. In July, its membership reached 2 million users.

"There are different reasons people use Foursquare," said a spokesman for that company at the press conference. "Our badges, points system and mayorships" offer added benefits. Foursquare, he said, will continue "innovating" with its program.
Facebook officials made a point of saying privacy will be protected with Places. The social networking site, with 500 million members worldwide, has long been under scrutiny for for its privacy practices.

A Facebook spokesman said Wednesday there will be a privacy "widget" for Places, so that users can choose which friends and families can be in on location sharing. But disabling the sharing feature will require that users go to Facebook's privacy page, and it's an extra step many may not think to do.

Not everyone is convinced about Places' privacy protection, and they didn't wait long to say so. While the Facebook press conference was underway, the ACLU of Northern California issued a statement against the new program.

"Facebook made some changes to its regular privacy practices to protect sensitive location-based information, such as limiting the default visibility of check-ins on your feed to 'Friends Only.' But it has failed to build in some other important privacy safeguards," the organization said.

"In the world of Facebook Places, 'no' is unfortunately not an option. 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. But when it comes to opting out of that feature, you are only given a 'not now' option (aka ask me again later). 'No' isn’t one of the easy options."

Walt Mossberg of The Wall Street Journal wrote he has tested the new service, and "found it easy to use and reliable, with mostly logical privacy controls, an issue on which Facebook has been bruised in the past."

Jules Polonetsky, a former AOL executive who now co-chairs the Washington-based Future of Privacy Forum, said that "Following the flaps created by earlier launches, Facebook seems to be taking a more moderate approach to location sharing. Just about everybody recognizes location as something that you want to be in control of."

Facebook Wednesday emphasized that users will first need to "check in" on their mobile devices when they want to share their location, which is identified by the GPS chip in a mobile phone.