Snapchat has always been more about the one-to-one messaging. Yes you can do one-to-many, but it still gives a more personal feel than a many-to-many group message. You had to click on that person to send it to them, you actively thought about them no matter how instantaneously.

I use Snapchat for both very close friends and acquaintances too, I can send both inside jokes or just a simple hello to someone I haven't spoken to in a while. I don't feel an obligation to respond to snapchats and don't feel ignored when someone doesn't respond to mine, how I would via text, or facebook messenger.

I personally never use facebook messenger. I've had it turned off for a couple of years now and only use it on the rare occasion that a friend changed phone numbers (friends moving abroad mostly) and even that is just to get their new number so I can use whatsapp or text messaging. As for whatsapp I strictly use it for large, on going, group messages or friends in other countries. But I guess everyone has their own use-cases.