Citation Needed...?
But really, at that point, it's common knowledge. Kids these days are on Instagram and Snapchat. Take a look at this article. It's really light, but it should refresh a bit into how tweens and teens consume social content: http://www.buzzfeed.com/benrosen/how-to-snapchat-like-the-te...
They've already started monetizing and seem to be doing pretty well. The "Discover" channels are pretty expensive and drive a LOT of traffic (Buzzfeed gets 21% of it's views from Snapchat[1]). Additionally, ads are inserted into "live" stories and you can buy extra filters as well. As far as I can tell none of those steps have alienated their userbase. Of course, the teen/tween crowd tends to be pretty fickle, so we'll see if they can pull it off long term.
[1] http://www.businessinsider.com/buzzfeed-gets-21-of-its-traff...
Facebook has and always has been targeted at adults. It was for people post high school. It's for connecting to people not in your immediate geographic vicinity. Facebook was only ever "cool" for kids because the barriers to entry dropped and they could suddenly see what they were barred from. It was always a utilitarian tool that is what you make of it. Follow cool people and groups, its cool. Follow people who share debunked myths, it sucks.
It doesn't really matter where kids are as long as they sign up for facebook when they graduate highschool. The people using snapchat as their primary address book to stay connected to each other, are the people facebook will have a hard time winning over. to them, facebook will always be social network #2.
Zuck made it for himself and it has aged along with him. It's possible people will always sign up for it, but it's also possible that it becomes grandma's photo sharing site.