In 5 Things Instagram Got Right that Others Before it Couldn’t, Hillel Fuld outlines the following five reasons why he believes the iPhone photo sharing app, Instagram, has seen explosive growth:

1. True Social Integration – the usual cast of characters includes Facebook, Twitter, Flickr, ForSquare, and Tumblr. Fuld points out that while this is far from innovative, the ability to share on these platforms works and is dead simple.

2. Pictures on FourSquare – You snooze, you loose. Apparently, FourSquare users have been clamoring for photo sharing feature directly within ForeSquare. Instagram has delivered.

3. Utter Simplicity – In Fuld’s own words,

I cannot stress this point enough. If you are a developer, you might want to run a short analysis on all the apps that have gone viral recently. Angry Birds, Flipboard, Cut the Rope, Real Racing, and many more. The common thread that jumps out at me is simplicity. These apps are not overloaded with fancy features and they do not do extraordinary things, they just work.

4. Simple Photo Sharing – In a Mashable blogpost, Jolie O’dell argues why the social aspect of photo taking is so wildly important to people,

It sounds like an install-it-and-forget-it application, but that couldn’t be further from the truth; Instagram nails the social aspect and engineers an ego-driven, must-revisit experience as compelling as Twitter.

5. Scalability – How does one honestly project the scaling needs of an app? Apparently, Flipboard CEO Mike McCue claims to have doubled the amount of servers his engineers recommended for a go-live, only to have a complete server collapse within minutes of initial release. Learning from these kind of experiences, despite Instagram’s massive success (reportedly 100K downloads in the first week), he app has been responsive and robust.