From what I've heard, if you are 13-25 with a smartphone, there's a good chance your primary source of communication is snapchat. And unlike other platforms (WeChat, WhatsApp), snapchat has a content delivery mechanism. So not only do friends connect with friends, but they also connect with businesses, celebrities etc.
Twitter was never about fast communication between two people, no one every said "Let's see if Mike wants to join us for dinner, I'll just Twitter DM him". Whereas WeChat, Viber, Whatsapp allows you to communicate quickly, there's no connection to advertisers (from what I know). Therefore snapchat is valued so highly because it does what twitter and Whatsapp et al couldn't do.