Someone has replayed your snap! Instant gratification for the long term.
I am not the snapchat type. I prefer books, code, and UIs that let me drive, think about what I'm doing, and build things over time. But with two of my kids off to college and Snapchat their preferred method of communication, I'm snapchatting.
What strikes me about the app, is the emotional bang for the buck it provides. Low overhead - down in the tenth-of-a-second range - as well as nothing to organize later, nothing to store, and no definite expectation of response makes the 'cost' of using the app in terms of time, effort and responsibility almost zero. The gratification of getting a snap unexpectedly (usually they seem not to be part of an ongoing conversation) or of seeing that one of your friends or family has viewed or saved your snap is fairly high, especially when its a photo of a distant family member.
Spontaneity is another plus. Silliness is encouraged both by the app itself (filters, face swaps, etc) and by the sense of impermanence. One might hesitate to post a dog-whiskered, clown-nosed version of oneself to a online photo album, but the sense that this is like a voice conversation that will disappear, encourages fun
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