In 2010, in the USA, datacentre electricity use was 2% of the national total electricity use. Worldwide, datacentres use about 30 billion watts of electricity, roughly equivalent to the output of 30 nuclear power plants. This is probably on a rapid exponential growth rate (anyone know?) so while it's astonishing to see so much energy being used to power the computronium, it's also a huge opportunity for energy savings.
Data Centers Waste Vast Amounts of Energy, Belying Industry Image
http://www.theverge.com/2012/9/23/3377868/cloud-internet-infrastructure-waste-energy-new-york-times
http://www.forbes.com/sites/danwoods/2012/09/23/why-the-new-york-times-story-power-pollution-and-the-internet-is-a-sloppy-failure/
One stat, between 2005 and 2010, datacentre energy usage increased by 56 percent worldwide and 36 percent in the US. This suggests that efficiency is improving quite quickly since all other measures are probably increasing considerably faster.