Interesting trend that could be developing. By it’s very nature Facebook seems more suited to people aged 20+ who likely have diverse social circles of friends (school, college, university, work, family) rather than teenagers who are primarily focused on a single social circle (school). If you only have a single circle of friends then messenger apps make more sense.
As social media matures it could be we’re moving away from the ‘all-in-one’ Facebook model that includes status sharing, messaging, photo sharing, towards a more diverse range of platforms. Certainly Facebook has lost any trace of ‘coolness’ and has become another communication channel similar to email: