We all know how immensely popular Instagram has been, considering Facebook purchased them for $1 billion, but how have they gotten this popular? Speed. When sharing photos, you don't want to snap the shot, wait a few seconds for the app to finalise the shot, wait a few seconds, begin to upload and wait more seconds for it to upload, right?
Well, how do they do it? The image below explains it, really:
As you can see, as soon as you have taken the image, selected the filter and tapped the green tick, Instagram begins its upload regardless of whether you upload the picture or not. Instagram makes "two requests, two round-trips", where the data is sent as soon as part of it is ready to go, after which, it matches the rest of the data up. So what you think is a static screen with your picture, is actually magic behind-the-scenes, where it uploads your picture and appears to be super fast when it's finished.
This is most likely one of the key reasons Facebook acquired them for so much money, because they are great at what they do. As well as being a kick-ass photo-sharing app.