I have a phone number for everyone I want to snapchat with, but a couple of those people are not on Facebook. I can't imagine someone not having a smartphone in my demographic but not having FB, while rare, does happen.
Sure, but that requires users typing in their phone number and having Snapchat send a text to verify. I don't know about your friends, but I know for sure mine would not do that. In my opinion, Facebook is much more reputable than Snapchat.