I suspect part of the reason for the power difference is the rackspace design has power fitted to approx 1x208V 30A 3-phase circuit per zone. Since they use co-lo they may be billed based on the circuit size regardless of the amount of power draw, so it is beneficial to maximize circuit capacity.
Facebook might be using 480V 60A 3-phase which may be then split into 3x20A 277V to get ~4.4kW/zone (@ 80%) ?? sort of outside my realm of expertise.
If so - also likely then that Rackspace colos won't offer them 480V infrastructure to do something similar.