Last week, Facebook published energy usage figures for 2011, giving us all an idea of how much our social media chatter is costing the planet. The publication also gives plenty of insights into how much difference the company’s efficiency measures are making, and some directions for green data centers.

First things first. According to Facebook figures, each active user consumes enough energy per year, to make a single latte. So social media isn’t actually killing the world, or at least has an energy use wee can put n context with other activities.

But there is more, in the social giant’s information (the newsroom link is offline as I write, but the info is here).

In 2011, Facebook’s data centers and operations consumed about 532 million kilowatt hours (kWh) of energy, and emitted 285,000 metric tons of CO2 equivalent. That’s about a quarter as much as Google, which last year said it uses about two billion kilowatt hours of energy (though for a real comparison we’d have to know what was included in both figures, and consider the different scope of the two businesses).

Facebook reckons nearly a quarter (23 percent) of its energy comes from renewable sources, which is a credit to the company and compares with Google which has a target of a third. A further 13 percent comes from nuclear which is arguably clean since it produces very little greenhouse gas).

Greenpeace welcomed the publication - and so it should. The pressure group’s energy crusades are all about getting companies to be more open and stop treating energy and environment data as top secret.

And Facebook’s openness goes beyond what you might expect, publishing data for carbon footpring and even water usage, using some of the newer - and more experimental - metrics provided by the Green Grid.

WUE, or water usage effectiveness, measures the amount of clean water used per kWh of energy, something which should be of concern since the world has limited clean water resources.

Facebook scored 0.22 liters of water per kWh, which FAcebook’s Daniel Lee described as a “great result” in a blog post - though he points out that WUE is still in development, and there are no benchmarks. However, he did describe several means whereby Facebook is reducing its water usage.

One interesting side to the figures, picked up by DataCenterDynamics, could give the tech media pause to reflect. Massive publicity is given to new data centers - such as Facebook’s facility in Prineville - but 85 percent of its data center energy is consumed in colo space which Faceook doesn’t actually own.

Facebook is moving its resources into its own data centers, but will always have servers in shared spaces. Companies have less control over these sort of spaces, but Facebook is pushing retrofits and upgrades to the colo it uses. Putting in cold-aisle containment has saved it masses of energy and carbon footprint, reports DataCenterKnowledge’s Rich Miller.

That’s a lesson for other organisations: older data centers will carry on in use for a long while, and big new flagship projects shouldn’t distract the industry from retrofitting as much efficiency as possible to the existing sites.