If the forecast is correct, within four years almost half of Britons will belong to a social networking site such as MySpace and Facebook.
Datamonitor said that interaction is the key factor in a network's success and that while the trend is driven by the younger age groups, many older generations are also being drawn to social media.
It expects the UK to see the strongest growth in numbers in Europe over the next five years because its consumers have been quick to embrace the new technology and many social networks are only available in the English language when starting up.
Datamonitor points to several factors behind the strong growth in social networking sites including the convenience of being able to socialise without leaving the house and the popularity of user-generated content.
France recorded the second highest number of networking site users with 8.9m, a figure that is forecast to rise to 21.3m by 2012.
Germany came in third with 8.6m and is expected to rise to 21.7m by 2012. Overall Europe is predicted to more than double from 41.7m to 107.4m.
Matthew Taylor, consumer markets analyst at Datamonitor, said: "Consumers are creating and joining existing social communities which, if harnessed correctly, can be an important marketing tool.
"The wealth of information stored on online social networks provides many potential benefits for marketers."