wow.. ok well the number provided includes QUITE a few things other than servers.
Lets break it down..
Cooling. At least 33% of that quoted data center figure is probably cooling.
Switching is at least 3%
So there goes 36% of the power, if directly translated from the server count of 180,900, it leaves us with
121,203. Fairly significant reduction. And some of that has to be storage only devices, which SHOULD account for a lions share of the power consumption. However, this would depend greatly on the methodology used in creating the data center, as there are many ways to do it that would require more or less storage only devices. If, for some reason -- They decided to take the approach of purely distributed, most redundant, most expensive solution and use no SAN, and rely on direct attached storage per server, minimizing the loss of data per failure point (which is likely mirrored at this point), given the amount of users, and the, in the words of Chef from south park "Ridiculous load of pig crap", each user brings with them, I could easily see the total server count for FB being 121K using the methodology described above. If they use any SAN, it should be significantly reduced.