"Users who wish to announce their location to their friends on Facebook would tap a "check in" button to see a list of places nearby, and then choose the place that matches where they are.
"After checking in, your check-in will create a story in your friends' News Feeds [on Facebook] and show up in the Recent Activity section on the page for that place," Places product manager Michael Sharon wrote on Facebook's blog.
Facebook staff also said the feature will be useful not only to let friends know where they are, but also to learn if their friends are nearby at the same time, and to write down what they are doing at the location and what they think about it -- creating a trail allowing friends to see what the writer did there days or even years ago.
When users check into a place, they also can "tag" any Facebook friends who are with them, essentially letting other friends know that the person is there, too. To cut down on the potential for users to falsely tag friends to a place, people can tag friends only if the taggers themselves check in to the location, too."
on a-whole-nother note, or maybe semi-related, fb has changed the whole meaning of "friend." Regardless of whatever it meant before, now it means a (hopefully) amicable acquaintance who knows exactly or can access where you are, what you're doing, what you like, what you look like, what your friends look like etc etc aT ANY TIME THEY WANT! Also, even if you want your privacy protected, OTHERS can reveal it.
Good thing I have nothing to hide and LOVE abusing this new definition of "friend"