Dozens of apps buzz your phone when friends are nearby, and all of them are annoying. Highlight notifies you when someone nearby knows your sister, while Foursquare lets you know when a friend checks in at Walgreen’s down the street. An iPhone app called Now takes a different approach: what if you got notified only when friends were doing cool stuff nearby?

Now decides what’s hot based on where the most Instagram photos are being taken, and pushes you a notification when a friend nearby is taking pictures at a trending location. "If you want to know what’s happening in your city, go where people are taking the most photos," Now founder Ben Broca says. For example, yesterday I received a notification when my friend Tyler Gold shot a photo of an upside down car being filmed for "The Amazing Spider-Man 2." Now combines Tyler's photo with photos tagged at the same location so I could see what was going on from every angle. "That information is already on Instagram, but you don't have any context about what's actually happening," Broca says.

Instagram boasts more than 100 million users, many of whom tag their photos with locations. While these tags aren’t technically "check-ins," they may as well be, Broca says, and represent data just waiting to be mined. Instagram still provides no way to see what’s trending near you, or to search for photos tagged at your local concert hall. "The idea is being selective on which source we use," Broca says. "I think the correlation [between Instagram photos and what's buzzing] is very very high." Broca is also working to tweak Now's algorithms to keep them relevant. For example, Times Square might always boast 5,000 Instagram photo tags per day, but Now may only notify you when a parade comes around and that number jumps to 8,000.

While Now launched last May with the simple goal of highlighting popular places on Instagram, it's now finding a new utility as a means of seeing when friends are at exciting events nearby. Looking forward, Broca is confident that Now will find a place at SXSW, where mobile app startups do or die. Opening the app will show photos from the most popular bars and events, and will also push notifications to your phone if any of your Instagram friends take a photo at the same party you're attending.

Relevant, local alerts were part of Foursquare all along, but that service seems to have drifted away from finding your nearby friends. Now is trying to bring that back, but unlike other competitors is not trying to create an entirely new social network to do it. The app could very easily encompass updates and trending signals from other social networks like Twitter and Facebook, but Broca is satisfied with Instagram — assuming its API remains accessible, that is. "[Instagram] is rarely wrong," he says. "If people who don’t know each other take pictures of the same thing at the same time, there has to be something going on."