A small survey study inspired by this informal interview then found that teen respondents (and those age 19-25) favored Tumblr, a platform that is known for it's stream-lined look and topic-focused blogs (though it also reported that Facebook came in second in popularity).
How does this compare with overall numbers from Internet users ages 18 and above? A 2012 Pew report shows that only 6% of adult internet users use Tumblr, 13% use Instagram, and an impressive 67% use Facebook.
In addition to which platforms some teens are using, the December 2012 interview reported that teenagers' pictures on Instagram (Josh Miller was allowed to view his sister's pictures) consisted of people, while those he observed in his own feed (both on Instagram and Facebook) focused more on objects and scenery. This suggests that teens are more interested in the phatic "social" aspect of these platforms rather than incorporating and sharing other aspects of life like Facebook tends to encourage.
Facebook, wanting to be a hub of communication and sharing in general, is now full of political views, retail offers, social causes, religious musings, etc. It seems that this may be more of the reason teens are shying away. Not that adding adults to the participation format of a site necessarily makes it "uncool" by default, but that the content they produce and recycle does not match the social content sought by teenagers.
In conclusion, they're not great at explaining why (and it can make you feel like the "what" guy at the end of this commercial) but teens have different social media preferences than adults.
(awesome werewolf drawing taken [and adapted] from http://playinggodwithmonsters.files.wordpress.com/2013/01/werewolf.jpg)