Actually Instagram was iOS only at the announced time of acquisition. It was a pretty big bet at the time and almost no one believed it was worth $1B.

Some context for those interested:

https://www.techemails.com/p/mark-zuckerberg-instagram-fours...

You know who else knew how many devices Instagram was on? Instagram. And yet they were willing to sell. There is no conspiracy here. There is nothing nefarious. Facebook made a good bet.