The service will "help people share where they are in a really nice and social way," Facebook CEO Mark Zuckerberg said.
"It can help people stay connected not just at home but everywhere they travel in the world."
Zuckerberg said he knew the service was ready to launch when he was showing a prototype to his girlfriend while dining in a Menlo Park restaurant and they could see that a Facebook colleague and his fiancée were dining at a nearby restaurant.
"It was at that moment, when that kind of serendipitous thing happened, that we knew the product was ready to go," Zuckerberg said.
Zuckerberg was joined Wednesday by executives of other place-oriented Web companies — Foursquare, Gowalla, Yelp and Booyah — who said they were pleased to have the social media giant enter the arena. Facebook has more than 500 million active users, according to its website.
"This is a great thing for the still-small location-check-in industry that Facebook is entering this market because it validates that we're onto something, that we're actually adding value, that this will be a much bigger thing going forward," Gowalla Chief Technical Officer Scott Raymond said.
But not everyone "Likes" the new feature.
In a statement, the ACLU of Northern California said the new "Places" feature does not go far enough to protect privacy.
"Facebook made some changes to its regular privacy practices to protect sensitive 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.
"'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."
Zuckerberg and colleagues stressed the friendlier aspects of the new service, even suggesting the new "Places" technology can bolster civic engagement by restoring the importance of what sociologist Ray Oldenburg calls the "third place" — public gathering spaces.
"What we're doing is keeping the 'third place' alive and well," Facebook vice-president Chris Cox said.
"Technology is the thing that pulls us away from the TV and out to the nightclub, concert, theater or bar. Technology does not need to estrange us from one another."
Cox said Places can help friends find one another not only in real time — such as at a crowded Lollapalooza concert in Chicago — but over decades.
"Our collective stories are going to be pinned to a physical location," he said.
"Maybe one day in 20 years our children will go to Ocean Beach in San Francisco and (the technology will tell them), 'This is where your parents had their first kiss. This is the photo they took afterward, and this is what their friends said about the photo."
The announcement came in what was billed as a "press event" but which morphed into a "launch night" at Facebook's California Avenue headquarters.
Reporters who turned up at 4:30 p.m. waited 30 minutes before being asked to board shuttle buses to a different Facebook location. There they were ushered into a large lounge and joined by hundreds of others, including Facebook employees and partners.
Zuckerberg finally took the microphone at 5:25 p.m.
"It's great to have everyone come together and have a launch night," he said.
"This is going to be a really fun and interesting summer. We have a lot of new products that we're building and that are coming out.
"We have a lot of other interesting launch nights coming up this summer."
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