“So, at any point of time, overall Facebook power consumption equals:
(10,000 x 250 ) + (300,000 x 120) = 38,500 Kw or 51,629 HP
[...]
Hence on an estimated ballpark - Facebook daily consumes 924,000 KiloWatt-Hours with per capita of 3.08 KWh . Annually the per capita figure would be 1124 KWh /year or equivalent to emission of 0.75 ton of green house CO2 (Carbon footprint) which is half of NY city’s carbon footprint.”
In trendsspotting.com, 10 Billion Facebook Photos - Is That Ecologically Sustainable?