I think teens nowadays are much more guarded (given how long social networks have been around now). They've grown up with the knowledge of how social media, when misused, can come back to haunt them. Most of my nephews and nieces have multiple throwaway accounts -- some of which they share with their parents and other accounts that they use as their main one.

Decreased FB usage is likely a reflection that they're wary of how FB utilizes their data, and realizing it's better to be choosy where they post info about themselves.