A year or two ago I saw my little brother and sister on Snapchat (16 and 15 respectively). I didn't understand it at all. Soon enough I was on it and let me tell you, it is used far more than Instagram and if not more, the same as Whatsapp. It is because you can share real moment of your normal everyday life. They mentioned how on Instagram most of the pictures they see there are uploaded celebrities/funny accounts/etc. Normal people don't have amazing moments to share everyday. Plus, they would be embarrassed to share what they share on Snapchat in any other network (i.e. funny faces, what you cooked, the funny kid on the school bus, your boredom).

I believe they are in unchartered territory thus comparing it to anything else is just a shot in the dark. If you could wave a magic wand and ask for the best way to share moments of your life you would probably get a hologram of your friends to show them that instant of you life that you wanted to share. Snapchat gets closest to this. Moments come and go, not all moments should be forever saved, moments should be only for those you intend to share with, moments should be feel lively (video/images) not text, etc.

Also: Discovery feature - I give Snapchat so much credit for this. They got the younger demographic watching 'TV' again. Snapchat accommodated the channel these kids are using (mobile) to connect with the big media outlets in a way that they actually consume content. That is, by short clips of < 5-10 minutes.