Facebook Inc., (Nasdaq: FB) owner of the world’s largest online social network, derived almost one-fourth of its energy for data centers from clean and renewable sources last year.

Facebook’s energy mix was 23 percent green, 27 percent coal, 17 percent natural gas, 13 percent nuclear, the company said Monday. While the remainder was uncategorized, it can include all the above energy sources and is available for purchase on the spot market, according to a report from the Menlo Park, California-based company.

[Facebook operates one of its largest data centers in western North Carolina.]

Greenhouse emissions or the “carbon footprint” from data centers, offices, employee travel and other factors was 285,000 metric tons (about 628.3 million pounds) of carbon dioxide equivalents, according to the report.

“We’re releasing this data because we believe in the power of openness, and because we hope that adding another data point to our collective understanding of our industry’s environmental impact will help us all keep improving,” the company said on its site. “We recognize that this data is just one slice of our overall environmental footprint, but we think it’s an important starting point.”

The company wants to get at least 25 percent of its energy for data centers from clean and renewable sources by 2015 even as it grapples with a growing user base that’s reached more than 950 million members. The energy mix may get “worse before it gets better” as Facebook grows, the company said. A data center in Sweden, slated for 2014, should boost the percentage of cleaner power, according to the report.

Rare ‘Transparency’

“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,” said Gary Cook, a Greenpeace International analyst.

“Unfortunately, the transparency Facebook exhibited today is still rare among companies who are racing to build our online world.”

The company’s data centers and offices used 532 million kilowatt-hours of power last year, while Google Inc. used 2.26 million megawatt-hours in 2010, according to a separate report from the Mountain View, California-based company.

Facebook estimates that each user on its service is responsible for about 269 grams of emissions, or the equivalent carbon footprint from a couple of glasses of wines or a medium latte.

Apple Inc. estimates the company was responsible for 23.1 million tons of greenhouse gas emissions in 2011, according to a report. Google estimates 1.46 million tons. Two percent of Apple’s carbon footprint comes from facilities, the company said.