The financial services search and save website increased its UK audience by 1,165% to 392,000 users last year, according to Nielsen Online.
Social networking website Facebook grew its UK audience by more than 781%, taking its total users up to 8.9m last year, from 1.1m in 2006.
Alex Burmaster, Internet Analyst at Nielsen Online, said: "Many will be surprised to see that Facebook wasn't the fastest growing site of the year, this honour went to price comparison site BeatThatQuote -- a reminder that the internet provides valuable services other than virtually poking friends and acquaintances."
Google Search was the most popular UK website by a considerable distance, with an average monthly UK audience of 25.1m users.
Microsoft's MSN portal and Windows Live Messenger services came second, with 14.7m users. Ebay took third place with 14.5m users.
In terms of site activity, MSN and Windows Live Messenger came out on top, with UK users spending an average total of 2.8bn minutes a month on the websites.
Ebay was the second most engaging website, with UK users spending 1.7bn minutes a month on the site. Google Search polled third with 0.8bn minutes.