Facebook’s data center costs have more than doubled in just over a year, according to estimates.

Data Center Knowledge reports that Facebook is now spending $50 million a year to lease data center space, compared to $20 million in May 2009. The leap in costs is not surprising given that the social networking site has grown from 200 million users worldwide in mid 2009 to 500 million now.

Over the past year, Facebook has added multiple data centers in Ashburn, Virginia and Silicon Valley in California. The company has also announced plans to build its first wholly owned facility in Prineville, Oregon, which is currently expected to span more than 300,000 square feet.

The Data Center Knowledge estimate doesn’t include the cost of server and storage hardware or electricity to power them. It’s thought that Facebook is running at least 60,000 servers in its data centers.

Facebook’s costs remain substantially lower than the billions spent by Google or Microsoft. One possible reason, according to the Data Center Knowledge analysis, is that Facebook mostly leases its data centers, with the exception of the new Prineville facility. The wholesale model means that Facebook is spared the capital expense of construction but the rents carry a premium compared with regular office space.

The fact that Facebook is investing in building its own facilities shows that the company is planning for the long term. It guarantees that Facebook will have servers when it needs them, but any savings would only be realized over time. I imagine that, as Facebook grows, it would need to continue to rent data center space as well as building and managing its own.