Facebook's comments on its carbon footprint, which were posted on its website, come amid calls by green groups for the IT and social media sectors to slash their use of coal-fired electricity as global demand skyrockets for online data.
Facebook to prioritise CO2-free energy to meet 2015 target
"We've set a company goal to derive at least 25 percent of our energy mix from clean and renewable sources by 2015. We know this is going to be a stretch for us, and we're still figuring out exactly what it will take to get there," the company said.
It added that it would give a preference to locations that have access to clean and renewable energy sources when deciding where to base electricity-hungry data centres.
Facebook said by 2014 it will commission a data centre near the Arctic Circle in northern Sweden that will use 120 MW of hydropower, and the facility will use cold air funnelled from outside in order to cool down servers.
But the company acknowledged that until then, "our carbon footprint and energy mix may get worse before they get better".
Facebook's share of renewable energy is likely to fall in the next two years because of forecasts of increased demand for electricity at the company's two current data centres at Prineville, Oregon and Forest City, North Carolina, where coal-fired power dominates the energy mix.
The social media company, which has 950 million users worldwide, said 23 percent of its electricity came from renewables last year; 27 percent from coal; 17 percent from natural gas, and 13 percent from nuclear.
It added that the remaining 20 percent was "uncategorized", meaning it has used electricity bought by utilities on the spot market from multiple sources.
Facebook said carbon emissions from data centres, office space, employee travel and commuting, and construction of new facilities amounted to 285,000 tonnes of CO2 equivalent, while the annual carbon footprint of each monthly active user of the company's website is 269 grams.
"To put this number into context, one person's Facebook use for all of 2011 had roughly the same carbon footprint as one medium latte. Or three large bananas. Or a couple of glasses of wine," the company said.
Environmental pressure group Greenpeace in 2010 called on Facebook's users to "unfriend coal" as part of a campaign to persuade the social media giant to make the switch to renewable sources of energy.
Greenpeace last year ranked 10 of the world's biggest IT, social media and technology companies on the basis of their energy use, found that Facebook was the second-most coal-intensive behind Apple Inc
But on Wednesday Greenpeace praised Facebook's efforts.
"Facebook has committed to being fully renewably powered, and today's detailed disclosure and announcement of a clean energy target shows that the company means business and wants the world to follow its progress," Greenpeace International Senior IT Analyst Gary Cook said.
Greenpeace said technology companies such as Facebook, which are building huge new servers to store and process data amid surging growth in cloud computing, needed to be increasingly accountable for their energy use.
"The aggregate energy footprint of the cloud would already rank fifth among countries in terms of electricity demand and is expected to triple or quadruple in the very near future," Cook added.