The £300,000 campaign, running across the London Underground network and at Network South-East railway stations, quotes panel-sample figures from internet monitor Hitwise for July to September 2006.
The activity, by Clemmow Hornby Inge, is the first specifically promoting the site.
The ASA has received one complaint about the ad - believed to be from a rival newspaper - on the grounds that the figures are misleading. The regulator has not yet decided whether to investigate.
The campaign places the thorny issue of newspapers' online circulation figures in the spotlight and reflects a lack of clear consensus over which measurements to use. Ella Smillie, media and advertising manager at ISBA, said the lack of user-driven data in the industry is creating uncertainty for advertisers. 'As long as there are multiple currencies, it is confusing for advertisers, as it is natural to pick the system that puts their product in the best light,' she said.
According to ABC Electronic, in November, Guardian Unlimited had the highest number of users, 13,841,182, followed by the Times Online with 9,028,963 and The Sun Online's 7,578,042. Tele-graph.co.uk had 6,374,362.
A spokeswoman for Telegraph Group said: 'According to Hitwise, which measures the number of times a website is visited each month, Telegraph.co.uk is the number-one quality newspaper website in the UK, receiving the greatest number of visits in the audited period.'