I’ve been playing with Sonar of late and it’s definitely worth a look. You start by connecting your Foursquare account and have the option of also adding Facebook, Twitter and LinkedIn. From there you can check in to a location and/or track trending venues to see who is there, but also how you are connecting to people at any given venue. Sonar will let you know if you’re connected, within what network, and if you’re not it will let you know who you have in common based on your connected networks with others that have checked in. This is a feature I would have loved to see Foursquare integrate themselves.