Snapchat isn't really direct contact, you don't know if she is snapping just you. She could be sending the same snap to many different people. And a girl who uses snapchat probably snapchats a lot of different people. When asked to snapchat with someone, I'll do it, but I won't really promote it, since you won't really stand out using it.
I would casually get it and respond whenever they snap you, but focus more on one-on-one interaction.
Snapchat is a good source for nudes though
If they send you something, they expect a picture in return. If you are humorous, just take pictures of funny things and caption them. I think just taking selfies with no captions are weird and awkward, maybe have a goofy face or be doing something and caption it.