I was highly skeptical, but as of the past few months i use it more than FB and Instagram.
What I found was the limited and spontaneous nature of snapchat works well because it is much less formal and easy than a Facebook or Instagram post that I have to "compose".
I.e, i can take a silly picture of my dog and put a sticker on it and fire it out there, without having to worry about shot composition/hashtag use (Instagram), or content/grammar (FB)