As location-based checkin services have gained steam amongst early adopters over the last couple of years, many users have discovered that they can lead to serendipitous interactions between friends that wouldn’t otherwise have occurred. For instance, if you’re at a coffee shop, and a friend checks in at a store nearby, it might be a good opportunity to walk over and chat for a minute to catch up, or maybe grab lunch together.

So far, only relatively small numbers of people have used such services – in the range of a few million. Now, Facebook is giving location-based functionality throughout its platform, giving its users in the United States the opportunity to share their location with their friends, and see which friends are nearby. Facebook’s rollout of Places will drastically expand the “check-in” market, and make the serendipitous saying-hi-to-a-friend-nearby experience a much more common part of everyday life.

Of course, it will take some time for everyone to learn how Facebook Places works, and for people to collectively determine what the implicit social norms around such a service will be.  And Facebook is only rolling out the full set of features to US users for now. But Facebook, for the first time, will be introducing these features to the mass market – not just people who like trying out the latest apps in the app store. It will definitely lead to some interesting interactions – and opportunities for developers and marketers – in the months ahead.