The page view has been the traditional measure for advertisers to compare which websites provide the most opportunities to display their ads.
However, it has always discriminated against sites where content is consumed seamlessly - such as those rich in games, video and flash.
Consequently, advertisers have to look at other metrics, such as time spent or visits, to see where their online ad pound should be spent.
Visits measure the "stickiness" of a site, or people's loyalty towards it, but have a major gap when measuring user engagement - time. Distinguishing whether someone visited a site for one minute or one hour has a huge impact on the ad opportunity.
Of all single metrics, time provides the deepest measure of engagement and is the most reliable as a proxy for ad inventory - even though it can be a misleading metric for search or comparison sites that aim to get people off their site as quickly as possible. Time, however, covers all web environments and provides an accurate trend in a pre and post-Web 2.0 world.
So where does time indicate to advertise? In February, eBay (28 million hours), Google (22 million hours) and MSN (16 million hours) racked up the most total time spent by Britons online. However, among the 500 most popular ad-supported sites, adventure game RuneScape (6 hours 32 minutes), teen community Habbo (4 hours 11 minutes) and games producer Electronic Arts (3 hours 7 minutes) had the highest average time per visitor.