As widely expected, Facebook this afternoon announced its new location-based service, dubbed "Facebook Places," allowing its users to check in to and "like" specific locations. The service is available initially via an update to the social network's iPhone app.

One notable feature: Users will be able to check in their friends, not just themselves. That generated some interesting discussion this evening at the end of the first day of the pii2010 privacy and identity conference in Seattle, as news of the feature spread through the crowd. Speakers called it a signal that location-based services are becoming more of a "tribal" activity, not just an individualistic pursuit.

Privacy, of course, is a central issue. Facebook says people will be able to block themselves from being checked in to locations by their friends. But privacy advocates are already warning Facebook users to be cautious. See, for example, this in-depth post by the ACLU of Northern California. Here's a key excerpt from the post.

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. And if you use Places yourself, you aren’t even given a “not now;” you’re just told that friends are able to check-in for you and left to discover for yourself that you can change this setting by digging into your privacy controls.

Walt Mossberg of the Wall Street Journal also digs into the privacy issues in this review of the Facebook Places service based on his preview and early testing. Bottom line, he concludes, the privacy controls "are decent, but could be a bit better."

By default, the company says, people who use Facebook Places to check themselves in to locations will be sharing that information only with their Facebook friends.

Writes analyst Michael Gartenberg in his first take on Facebook Places, "Extensive details for privacy control show that Facebook spent a lot of time doing their homework on this effort that will likely allow them to avoid the negative repurcusisons that affected Google with their launch of Buzz.

The rollout of the Facebook service is widely seen as the company's answer to Foursquare and other existing check-in services. The rollout is being closely watched by startups including Redmond-based Glympse, which offers its own location service on Facebook already.

Update: Here's a Facebook video explaining the basics of the new feature, including the way it connects friends who are in close proximity.