I thought the social network space was filled up by the likes of Facebook, Twitter, Google+, kind of like how online bookstores have been dominated by companies led by Amazon. Sure, there are new social network competitors, but we grok the beast. But just when LinkedIn seemed to define the boundaries of social networking, Instagram comes out of nowhere and suddenly we understand less than we thought. Far from set in its ways, of social networking is being rocked by waves of game changers. In an article in Fortune, HootSuite CEO Ryan Holmes serves up 7 contenders to watch in 2013.