Middle school students I talk to frequently point to Instagram, Kik, Snapchat, WhatsApp or (increasingly) Twitter as their preferred social network over Facebook:
Teens, Social Media, and Privacy | Pew Internet & American Life Project: “Focus group discussions with teens show that they have waning enthusiasm for Facebook, disliking the increasing adult presence, people sharing excessively, and stressful ‘drama,’ but they keep using it because participation is an important part of overall teenage socializing.”
While Facebook did (wisely) acquire Instagram last year, the bleeding of usage is significant. Teens and the prized 18-24 demographic will continue to have a presence on Facebook as a namespace, but usage is the key demographic here.
Replacing demographics are much more ephemeral and harder-to-monetize situational networks like Kik or Snapchat.