Location-based social networks allow users to “check in” to restaurants, theaters, clubs—and your store. Here’s why you should check them out. Jennifer Lieu didn’t think twice about her store getting involved with the social media phenomenon that is Foursquare. “We as a company are very proactive and want to get the word out,” says the marketing director of the Wedding Ring Shop in Honolulu. “We’re on Twitter, Facebook—everything.”But she likes Foursquare in particular because when someone “checks in” to the Wedding Ring Shop via mobile phone, the shopper’s status i