Snap Jumps in Debut After App Maker Raises $3.4B in IPO 9 years ago
Sure the bulk of their users are young, but several of the people I follow are betweeen 28 and 45.
It's not like Snapchat is an esoteric thing. Using it can be a function of how outgoing and expressive you are. Or if you have some purpose.
The same reason you post on Hacker News is why people post on Snapchat -- to interact with others.