It's probably a bit early to start chiseling Facebook's tombstone, but according to a Mediapost report Tumblr is actually used more than Facebook by 10th graders.
"Tan used Survata to poll 546 teens ages 13-18 and 492 young adults ages 19-25 about their social media usage. Tan found that while 55% of the younger age group said they use Facebook, 61% said they use Tumblr. Meanwhile 22% said they use Twitter, 21% use Instagram, and 13% use Snapchat."
That number is very surprising to me - I had no idea that Tumblr was getting that kind of teen traction. Twitter use in that demographic seems to be increasing, too.