Like every other nascent social network, Twitter's ads sell because of the user data it's gathering. You can see promoted tweets based on who you follow, what TV shows are on, or what city you're in. But Ad Age reports that Twitter will soon add one obvious element: ads that appear when users are near a corresponding store. Sources have said that Twitter will let retailers target their tweets to users around a specific geographical point, a big jump from the city-wide options introduced last year. If you're around a McDonalds, for example, a promoted tweet could tell you about the specials.

Facebook already sells ads by zip code, and Ad Age speculates that Twitter will start with this and then move on to more specific location-based tools. The more obviously a promoted tweet pinpoints where you are, the more it blurs the line between advertising and feature. Google, Foursquare, Yelp, and many others have already turned finding local discounts into a business, and retail stores like the Gap have offered coupons to people sitting next to a physical ad.