It's a mathematical equation. They had an existing email list from Burbn of a couple of thousand users that they emailed when they launched Instagram. These us

It's a mathematical equation. They had an existing email list from Burbn of a couple of thousand users that they emailed when they launched Instagram. These users were extremely targeted because they already knew the company and because they were interested in photo sharing apps. Then the viral co-efficient was a lot higher than 1, maybe something around 3, because it was a good product. Users used the app a lot and told their friends about it. Then you add social sharing on facebook and twitter and you have enough growth, simple. :)