But isn't instagram (and also Snapchat) mostly photo sharing? How can that replace a communication tool? Do young people really like chatting in public comment

But isn't instagram (and also Snapchat) mostly photo sharing? How can that replace a communication tool? Do young people really like chatting in public comments and in that cumbersome Snapchat chat? Mostly I've seen teen girls just writing and "luv ya bb" under each others photos...