Location-based social networking on the iPhone is a tough game to play, apparently -- the popular Yelp app [iTunes link] has recently updated to version 4.0, and along with a friend finder, hooks to Facebook and Twitter, and an updated version of their augmented reality Monocle feature, they've added the ability to "check-in" from locations around whatever city you live in. When you visit a great pizzeria, for example, you can send a ping out that says you've been there along with what you think of it. This replicates the functionality of another app, Foursquare [iTunes link], which is steadily building up its own social network prowess (you may have seen the "mayor of" tweets lately -- that's Foursquare doing its thing, either on iPhone or through a desktop interface).
In this case, the imitation isn't appreciated -- Foursquare co-founder Dennis Crowley tore apart Yelp's new feature in a recent tumblr post, both comparing the two extremely similar "check-in" systems to each other and claiming that Yelp "copied the wrong stuff," and that Foursquare is already working on improvements to their own system.
Which is the good news for us consumers, as competition tends to breed some great innovation. Personally, I haven't been too interested in an app that helps me broadcast my location everywhere just yet, but there's no question that these are popular apps and their popularity is growing. We'll have to see what new features Foursquare has planned and how Yelp eventually responds.