Snapchat's main draw seems to be amusing filters for selfies. I have a bunch of friends* who use it for that but do most of their actual socializing on FB. Snap is like waving at people you know in the street, FB is like sitting down with them for coffee.

* anecdotal but I'm taking demographics into account. I've asked a lot of YA friends about their usage patterns, connectedness etc. on Snap and other social platforms.