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They paid a billion dollar to prevent Twitter from having a decent photo service. Facebook doesn't need Instagram. Twitter did.

Those are daily active users. Monthly active users are around 43.6 million.

That's more than 1% of Facebook's users. At the time, Facebook was worth 100B.

But also consider that many if not most of those users were already on Facebook.

And they had reason to LEAVE facebook (or at least, stop being so active on it, and concentrate on twitter).