Bebo.com is ranked second by Hitwise with a market share of 25.04%. In the US it is ranked 5th with just over 1% of market share down 41% on last year.
MySpace.com is ranked third with a market share of 14.75%.
Together, the big three social networking sites account for just over 85% of the UK social networking market leaving only scraps for smaller rivals.
ITV's Friends Reunited features at five, just below Microsoft's Windows Live Spaces, but both have just 3% of the market between then with 1.5% apiece.
Top 10 social networking sites in the UK, June 2008:
1. Facebook 45.29%
2. Bebo 25.04%
3. Myspace 14.75%
4. Windows Live Spaces 1.59%
5. Friends Reunited 1.55%
6. Club Penguin 0.83%
7. Yahoo! Groups 0.81%
8. Nasza Klasa 0.75%
9. Faceparty 0.74%
10. Tagged 0.72%
Bubbling under there is major growth for micro blogging site Twitter, which has seen its UK traffic soar by 485% this year, 70% higher than the US.
Hitwise found that weekly UK internet visits to Twitter.com increased by 631% during the 12 months ending July 12, with 485% of that growth coming this year.
Twitter is currently more popular with British users than Americans. The site's share of UK internet visits was 70% higher than its share of visits in America.
Robin Goad, research director at Hitwise, said: "Although it is still most popular with classic early adopter segments of the population, the breadth of Twitter's appeal may surprise some people.
"During the four weeks ending 12 July 2008, visitors were split 50/50 male/female, while only 15% were from London. 25-34 year olds are still the most over represented age group visiting the site, but 37% of visitors to the site are now aged 45 and over."
Another sign of maturity is that mainstream media organisations are starting to pick up traffic from Twitter. For example, BBC News now receives more internet traffic from Twitter than it sends there. The BBC Sport feed is also very popular, and during the four weeks ending July 12 2008, BBC Sport was the second most-popular search term sending traffic to Twitter.
In the US, MySpace's dominance is even more pronounced than Facebook's in the UK. It has 71.92% of the market, with Facebook in second place with 16.91%.
This stark jump demonstrates that the site has not managed to achieve the widespread acceptance beyond university students that it has in the UK. However, its growth does represent 40% growth over the previous year.
Top five social networking websites ranked by market share YoY change:
1. MySpace 71.92% (-6%)
2. Facebook 16.91% (40%)
3. myYearbook 1.54% (318%)
4. Tagged 1.08% (45%)
5. Bebo 1.05% (-41%)