A new iPhone app called Highlight is poised to be this year’s breakout hit at South by Southwest, the Austin tech and media conference that has become known as a web service kingmaker after launching services like Twitter and Foursquare to a wide audience in years past. In the context of a major tech conference, Highlight makes an appealing promise: let it run in the background of your phone, persistently collecting your location data, and it will notify you when your friends, their friends, or people with shared interests are nearby. Highlight is only the most prominent in a collection of apps offering this sort of “ambient social networking.”