You don't just look at Instagram when you evaluate the acquisition, you look at Facebook also.

Facebook was huge, and Instagram didn't represent anything Facebook couldn't just build themselves, other than the growing user base. So there is your rationale, the behemoth shouldn't be completely free to consolidate users. Of course the details of how big is too big and so on are the hard part.

I think that's the point. If it wasn't anticompetitive then, why is it now?