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How do you account for the success of Instagram, which has been iPhone only for over a year?

Is it their API that has enabled web-based add ons? Or the simplicity of the app and the ease of use?

If you click through any of these links: https://twitter.com/#!/search/instagr.am

You'll be taken to a rich web experience that allows even non-iPhone users to engage and in turn share with friends via Twitter.

So Instagram has done both the web and iPhone really well which has been enough for them.