Because you got into Facebook so late.
People who are 20 are doing on Snapchat today what I did on Facebook when I was 20: multicasting. I either want to send a message to everyone or to a group. Sometimes in a personal message. I see almost no real difference in content. Snapchat might be a little racier in personal messages, but that happened on FB and Twitter as well.
The ephemeral nature was built into Facebook and Twitter as well, to some extent, as the services weren't searchable. You'd post something and after a few more posts it was essentially "gone from view".
Outside of marketers and people trying to "build an audience" I really see almost no difference at all in the way people use it. It's just a matter of how old you are and what service was popular at the time.
Don't believe me? Do this as an experiment: Create a new account on Facebook, Twitter, Instagram and Snapchat. Follow only 30 real people. No more, no less. 30 people you know. No marketers or self promoters or any of that nonsense. Just follow 30 real people, maybe a celeb or two the way a 20 year old might. Look at the content. It's all the same stuff. Having fun here, doing this with person X, Y, and Z. Cute dog photo. Inspiring quote. Maybe a DM with some nudes. There is nothing to "get" other than to understand that as you become older the thing that's changing is you. The biggest difference between Facebook, Twitter, Instagram and Snapchat is how old you and your friends were when you started using it, and what what point in the lifecycle of the service you were at.