Making use of Facebook's recently-introduced Places feature, when a user checks in on either Facebook or Foursquare, it alerts Meet Gatsby, which will find others nearby with similar interests. Then (as before), it sends both parties a text message telling both users about each other, then provides a link for them to meet up.
In an e-mail to CNET, Meet Gatsby's co-founder Will Tsui said that "Gatsby's matches have been getting better and more proximate as our user base has grown with Foursquare," and that the inclusion of Facebook will help make those matches more diverse.
The service still relies heavily on text messages, which is something to keep in mind if you plan on signing up and do not have a SMS package through your cell phone provider.
One thing I'd like to see in a future iteration of the service is better utilization of a user's Facebook profile information. Even when you sign up for the service with your Facebook account and give it access to your tastes, that information does not get pre-populated in your Meet Gatsby profile. So, for the time being, you still have to fill out all that information on your own. The good news remains that the service only needs three or four interests to begin finding its matches.