The two single guys in my circle of friends use it for the kinds of communication you wouldn't want to occur in the same app where your relatives are (say, Facebook/Whatsapp). One of them's outside the app's usual age demo but is definitely around them enough that it's a very valuable app for him (network effects—everyone at any given bar has it and you'll have much greater success connecting with them over Snapchat than anything else).
I don't know anyone else who uses it, but the rest of us are boring married people who don't go to bars much (and not for the same reasons if we do). A couple of the marketing guys at work send stuff out on it for the company, I gather, which is really weird to me since my impression is that it's (still) mostly used for sending clips of drunken stupidity and nude pics or hookup arrangements. They definitely don't use it to message grandma—that's what Facebook is for.
The vast vast vast majority of snaps are not nudes or drunken debauchery. Most of them are just mundane moments of life: I saw a building while walking down the street, or, here's my lunch.
Snapchat is useful and popular because it is a way for people to say hi to their friends without having anything to say. You just take a selfie with a filter, make a face, and you're done. This lets you feel close to your friends without having the pressure of actually having a conversation topic.
The genius of Snapchat is that because photos are ephemeral, the bar to create content is as low as it gets, which results in orders of magnitudes of more content creation and sharing compared to platforms like Instagram. On Instagram, you spend all of your time obsessing over the perfectly angled and filtered photo, hoping to get as many likes as possible. As a result, you'll post on Instagram once a week, but you'll post 5 times a day to snapchat. That's a two-orders magnitude of difference in usage, which is why Snapchat is so successful.
I don't know anyone else who uses it, but the rest of us are boring married people who don't go to bars much (and not for the same reasons if we do). A couple of the marketing guys at work send stuff out on it for the company, I gather, which is really weird to me since my impression is that it's (still) mostly used for sending clips of drunken stupidity and nude pics or hookup arrangements. They definitely don't use it to message grandma—that's what Facebook is for.