This sounds like one more step to making push something that's expected on the web. BlackBerry did it to email when they created the "interactive pager", but it took a few years before people realized what was happening.
It's great to see hot companies like Instagram jumping onto a coming wave like this to popularize it sooner.
This seems like a great idea with benefits for both sides. I haven't messed with too many APIs yet but are they the first to use POST requests this way?
I'll definitely try and hack something later tonight as I don't own an iPhone and would like to keep up with friends' instragr.am updates.
I dig the use of PubSubHubbub. It's very similar to some ideas I've been kicking around (I have an experimental/incomplete Node.js PSHB implementation). Take out all the stuff about hub discovery, feed content notifications, and xml, and you're left with a simple protocol for users to subscribe to updates on a given topic.
This doesn't seem all that different than Twilio sending you POST requests when you get a call or SMS. Very cool application of the concept though - I like it!
It's great to see hot companies like Instagram jumping onto a coming wave like this to popularize it sooner.