This started happening a few years ago. I was tipped off by my teenage daugther who told me that while a lot of middleschoolers were still facebook (a lot of whom didn't have phones yet but had computer access at home) that most high-schoolers had migrated to twitter. The reason was that too many parents were on Facebook and kids don't want their parents to know what they are doing. Very few parents were and are on twitter.
With two kids in middle school I can also say that within the last year instagram has far surpassed facebook for that age. There are also a couple of other chat programs being used that I can't think of offhand. But basically, programs that you can use on your phone/tablet but aren't on the computer are preferred because parents aren't that tech savvy and are less likely to be on those social mediums.
Facebook has become a medium of older folks. I bet the median age has skyrocketed in the past 5 years . . . probably doubled (true ages though, not the fake ages a lot of kids were putting on their accounts).