The measurement firm's rankings for January came with two new metrics--average daily unique visitors and total usage minutes--that are believed to provide a more accurate portrait of how well a particular site is faring, Media Metrix said.
In the category of time spent online, America Online ranked highest, with visitors spending a total of 21.6 billion minutes, or 38 percent of Internet time, on its sites in January. Microsoft was second, accruing 4.2 billion minutes, or 7 percent of all time online, Media Metrix's report said.
"The addition of these measures to our monthly syndicated reports will help advertisers and marketers make smarter and more strategic business decisions," Mary Ann Packo, Media Metrix president, said in a statement.
Tax information and advice services such as Money Central, IRS sites, Quicken sites and Turbo Tax saw a surge in traffic as the filing season gets under way. Turbo Tax, for instance, drew 1.4 million unique users in January, up from 424,000 in December.
Some tax sites have experienced problems, however. H&R Block's online tax filing service, for example, last week exposed some customers' sensitive financial records to other customers, prompting the company to shut down the system. The site has since come back online.
In the category of most-used digital media and Web properties in the United States, the AOL Network, which includes the company's proprietary service and Web sites, came in first, with Yahoo, Microsoft sites, Lycos and Excite@Home ranked in the top five.
The most popular single Web site last month was Yahoo, followed by MSN, AOL.com, Microsoft.com and Netscape.com.