PiperJaffray, a Wall Street firm that specializes in technology, reports that some less popular social sites such as Tumblr and Kik are seeing user numbers fall as teens aged 14 to 18 concentrate their social networking on Snapchat, Instagram, Facebook and Twitter. The research found that 81% of teens use Snapchat, 79% use Instagram, 56% user Twitter, and 51% use Facebook.