Yelp helps you figure out what is good in your area based on user reviews.  I should know, I actually Yelp my own reviews under Scott “SHANKAPOTOMUS” S in TX.  Yelp is cool, but I notice that, like other social network sites, there are a lot of “lurker’s” (those that read reviews, but do not post).   

Foursquare seems cool as well (disclosure, I do not have a smart phone) as it tracks visitors in real time when they “check in”  via their smartphone and it can sync with Twitter and Facebook, so Foursquare users are telling the world where they are “real time”.

What Foursquare can do over Yelp (although Twitter can do it as well) is tell others where you are and have them join you, thus increasing patronage.  I think that Foursquare users will check in more frequently (and write insider tips) than Yelpers write reviews based on that “lurker” phenomenon. 

This coupled with how Foursquare connects you with your address book “friends” and encourages usage with badges and rewards should benefit any business.