That last trend of people using Facebook less and Instagram more bolsters the justification for Instagram's large purchase. I wouldn't be surprised if Zuck can see that trend from their internal data, and the community at HN hasn't noticed it yet because it's happening with the younger generation first.
I was talking to my daughter (17) about this. As others have said, Facebook is out, and Twitter, Instagram, and Tumblr are in. The one comment about Facebook that stood out: "It's been taken over by moms."
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.
I thought the Facebook comment was pretty telling. I'm not even sure what FB's target demographics are, but the sense I got from the OP was Facebook is just a scene which was cool for a while but is now passe and they're moving on to other social networks.
It would seem FB for the younger folks has simply run its course as trend.
I would be surprised if he WEREN'T using his data to do that! He has age, location, social graph and referrer headers - I'm pretty sure he can use this to predict the next big trend at least a few months before the rest of us.