Are you sure? This document suggests GCE instances use the 10.x.x.x address space (just as AWS instances in EC2 Classic do):
https://developers.google.com/compute/docs/instances-and-net...
https://developers.google.com/compute/docs/instances-and-net...
For example, if you want to assign 10.1.1.1 specifically as a network address to a virtual machine instance, you can create a static network route that sends traffic from 10.1.1.1 to your instance, even if the instance's network address assigned by Compute Engine doesn't match your desired network address. """
Meaning they could have avoided conflicts using this mechanism.
At any rate, Instagram's been around since 2010 and GCE didn't exist until June 2012 (and wasn't generally available until this past December).