Yeah, no. With very few exceptions FB builds and owns their own datacenters. Basically, anything you read about in the news is entirely self-owned and operated. Yes, even employing security guards. E.g. this http://money.cnn.com/2017/08/15/technology/facebook-ohio-dat...
They also buy on the wholesale market taking over entire buildings and campuses. DFT had a great business model. Leasing out entire campuses to hyperscale players on 30 year leases before even breaking ground. Pre-determined interest rates means you know exactly how much you can charge per CRSF and make a good living. Honestly, I was pretty pissed when DRT bought DFT.
FB and et al do still build their own DC's. Especially where municipalities drop their pants and offer huge tax breaks. But there's still a tremendous amount of "single tenant" purchasing done at the wholesale level by the hyperscale players.
I guess my point is that even at the top tier, people value flexibility.