The service, a joint venture by America Online and Germany's Bertelsmann, hit 150,000 members in March and 200,000 in June. AOL has over 700,000 members in Europe and over nine million worldwide.
"We have achieved that through sticking to our three main principles -- ease of use, functionality and fun," said AOL managing director Jonathan Bulkeley in a prepared statement.
City worker Tracy Back won a year's subscription as AOL's 250,000 member. AOL said almost 20 per cent of members are female.