> Dumb question, but what is it about Instagram that made it so huge?

I'd like to think the quality of their app has something to do with it. Unlike almost every mobile app I use, it's a joy to use (Android version). It's always snappy (this is huge), things always flow well, there is a ton of little detail functionality in there but it never overwhelms.

It's also great to see people's posts for a location you've tagged in your shot and compare their experience to yours. Or just find them. I know no other social network that does this so well.

It's the only social network I've stuck with and gotten any joy out of.