Foursquare are quite controversial. They seem to be gaining in popularity, but at the same time many people avoid them as being a form of submitting to surveillance.
I think that the key attraction is that it forms a game. For example, the Foursquare user who checks-in somewhere most frequently in a 3 month period is elected as the mayor of that location. One time, I was very proud to be declared mayor of my local gym - even though I am sure many people went more frequently but just didn't check in on Foursquare. On the other hand, I was less thrilled to find out that I had become mayor of my local dentists.
Another key factor to remember is that checking in is voluntary. It requires a few clicks on your smartphone to check-in to somewhere and this is not something that I could do accidentally and reveal that I was somewhere that I ought not be.
One dangerous feature introduced by the new Swarm appp is called neighbourhood sharing. If you enable this feature it automatically reveals to your friends the neighbourhood that you are in. This is not quite as bad as revealing your actual location - for example if I revealed that I was in Castleknock I could be either working from home or dropping into my local pub for a quick pint during the work day. Nevertheless I recommend that you shouldn't enable this feature if you have concerns about your privacy.