The way web audiences are measured could be ripe for an overhaul, according to two reports out this week.
Britons spent almost 28 million hours on eBay during February
Measurements based on page-views and cookies (small text files which track net use) could be affected by changing user behaviour, the studies warn.
Net measurement firm comScore found cookies used to track user behaviour could be being over-counted.
A separate study argues that page-view measurements are outdated due to an explosion in audio and video content.
There are several methods for measuring net audiences which provide critical data for advertisers.
Cookies - small text files inserted on a user's computers by a web server and unique to that computer's browser - can be used for authenticating, tracking and maintaining information on users.
In comScore's study, an analysis of 400,000 home PCs in the US found that a hardcore minority of web users are clearing their cookies from their computers on a regular basis.
This causes servers to deposit new cookies which in turn could lead to an over-estimate of unique users to a particular website.
It found that 7% of computers accounted for 35% of all cookies, which extrapolated could mean the size of a site's audience is being overstated by as much as 150%, said comScore.
"It is clear that a certain segment of internet users clears its cookies very frequently. These 'serial resetters' have the potential to wildly inflate a site's internal unique visitor tally, because just one set of 'eyeballs' at the site may be counted as 10 or more unique visitors over the course of a month," explained comScore president Dr Magid Abraham.
Perhaps unsurprisingly, comScore offers a very different approach to audience measurement - using the panel-based system favoured by the TV and radio industries which relies on using a representative sample of net users to gauge behaviour.
TOP TEN WEBSITES BY TIME PER VISIT
RuneScape - 6hrs 32mins
Electronic Arts Online - 3h 07m
Bebo - 2h 37m
Facebook 2h 28m
eBay - 1h 55m
King.com - 1h 53m
Adventure Quest - 1h 35m
Fox Interactive Media (MySpace) - 1h 11m
Club Penguin - Ih 10m
Cartoon Network - 1h 09m
Its findings have led some advertisers and site operators to question current methodology.
"Cookie-based data are still a valuable resource, but this important study certainly underscores the fact that an accurate multi-dimensional picture of consumer behaviours must be compiled from multiple sources," said Jeff Marshall, senior vice president of Digital Managing Director at media agency Starcom USA.
A separate study by net measurement firm Nielsen/NetRatings questions how relevant it is to look at page views as a gauge of user behaviour in the light of new technologies and the increase in audio and video content.
The post Web 2.0 world is best represented by measuring time spent on particular websites, argues analyst Alex Burmaster.
Page-views metrics discriminate against sites with audio and video content and Nielsen/NetRatings argues that metrics based on the time spent on a website could be a more accurate method.
"As the technology that publishers use to deliver content to the user moves away from static, reloaded pages to more streamlined content such as online videos, the page view is becoming a less relevant gauge of where might be the best place to advertise online," said Mr Burmaster.
"Time spent is probably the best single indicator of user engagement although it can be a misleading metric for search and comparison sites that aim to help users find what they're looking for as quickly and efficiently as possible," he added..
While the biggest sites in the UK by page-view are Google and Facebook, measurement by the time spent on a site puts eBay and online game RuneScape at the top.
Britons spent almost 28 million hours on eBay during February, while visitors to RuneScape clocked up an average of 6 hours and 32 minutes per visit, according to Nielsen.