Interesting graph from Hitwise.

For the first time last month, UK Internet visits to social networks overtook visits to web-based email services. As the chart below illustrates, our custom category of the top 25 social networks, which includes Facebook, Bebo and MySpace, accounted for 5.17% of all UK Internet visits, compared to 4.98% for Computers and Internet – Email Services, which includes Hotmail; Yahoo! Mail and Gmail, amongst others.