Facebook was a social networking site.
Donna Noble compared the Sub-Wave Network to an "outer-space" Facebook. (TV: The Stolen Earth)
During the Atraxi incident in 2008, the Eleventh Doctor told Jeff Angelo to spread one word ("zero") across the whole world. Facebook was one of the ways the Doctor suggested. (TV: The Eleventh Hour)
In 2009, after the Torchwood Three team accidentally drove off the Severn Bridge, they heard a mysterious voice under the water. Back at the Hub, Ianto Jones stated that it was hit on Facebook. (AUDIO: Submission)