I feel like Snapchat is what all the younger kids are using, at least so I've noticed. I'm 30 but old school. I don't see them using it past a certain age (say.. college most likely). Everyone who uses Snapchat will probably gravitate to or concurrently use Instagram and Facebook. Facebook will persist into old age, perhaps Instagram as well.
There's a notion of active vs. passive engagement in each of these. Snapchat requires more active engagement to be interesting. There's the story aspect but not sure it's compelling enough. Facebook is nice because as soon as you turn it on you have this sort of mini magazine like feed that tells you what all your 'friends' are doing, you don't really need to do much to get involved, and it becomes easy to boop the like button to engage / like things.
What I think snap does have, that could be quite cool from a technology standpoint is its ability to create these "masks/goggles" that react when you say stick out your tongue or raise your eyebrows. I think that sort of what I would term reactive AI is something that carries a lot of potential. I don't know how much I see it becoming an ad-revenue generator, like FB/Insta/Twit that have more of this passive magazine like structure that still somehow fosters engagement.
Also curious to see what happens when rates go up