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Wow, they completely exposed their data set. Don't see many companies giving away cohort retention details like this.

The post was written by a 3rd party and not Instagram. The last section of the article explains how they managed to get the data:

"Four things allowed us to conduct this analysis: Instagram’s new API, their use of incrementing ID numbers to represent users and photos, the central limit theorem"

Unfortunately, this isn't very rigorous - the dataset can easily be gamed by instagram.

If the Instagram guys have read this http://en.wikipedia.org/wiki/German_tank_problem , they might be having a good laugh...

Well my point was more that Instagram exposed enough data such that RJ Metrics were able to get access to enough information to extrapolate the cohort retention rates.

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