It's by far the easiest way to share a picture or moment with friends -- a photo is worth 1000 words and the location/time/velocity overlays add a lot of context. In many ways it's actually one of the most mature popular social networks -- it clearly learned from earlier social network attempts. I don't have to be "cool" to anyone but the relevant people to whom I send a snap.
It feels private enough -- hot girls like to use it and that will get anyone on any platform/club/bar/social location. Sorry to all of you who are using other slower, more ad-ridden networks and missing out.