I've worked with youth in various churches the past decade, and it is very interesting to see the way they approach technology. They have perhaps a maximum of 3 years attention on any given technology, except a precious few that have always been there (email, SMS). As others have pointed out, right now Instagram and Tumblr are popular, and Twitter seems only popular for those near cities; in my rural area Twitter was only popular for maybe half a year or so before it died out. Facebook is still used, but you mainly see them posting stuff for family these days (not interacting with peers). Pinterest is very popular with the girls, but the guys get made fun of when they use it.
More interestingly, I'm recently seeing a love for Google again, especially with Hangouts. I wouldn't be surprised if this were the next big thing with teens.