Wikipedia and YouTube can be huge timesinks as well, but hardly see much commentary as regards to FB and Instagram.
Frankly when I was a kid there were oldschool forums I'd frequent all the time.
I think this isn't a "kid" problem. I see just as many adults glued to their screens.
Youtube can be informative as well, although the majority of it is not. I consider YouTube to be as much of a problem as FB/Instagram, with the caveat that it can, by design, have useful content (which the other two cannot).