Compete (a R/WW sponsor) has released data that shows MySpace and Yahoo are the top websites in terms of average time spent on them by users. The data is for US users only. Here are the figures from Compete, expressed as % of total time spent online in the US in Dec 06:
The good old long tail is in the house here, with 20 domains making up 39% of all the time spent online by US consumers. Another noteworthy figure is that only 2.1% of time was spent on Google.com (includes all sub-domains). But then Google is not - yet - a portal type service, unlike Yahoo which actively aims to keep users on their properties. This will probably change over time though because, as Rich Skrenta pointed out in his awesome essay, Google is poised to effectively take over the Internet. Or are they? See R/WW's point/counterpoint discussion for more on that theory.
Yahoo has received a bit of flack from bloggers and the press lately, but in terms of Web content it is still in a very strong (almost dominant) position. As Jay Meattle pointed out in the comments, Yahoo Mail is the #1 mail provider (much larger than Gmail), Yahoo Finance is probably the most trafficked Finance portal for stock quotes, Y! Answers has also now become the #1 answer service on the web, Yahoo also has Personals (personals.yahoo.com), Autos (autos.yahoo.com), Music (music.yahoo.com), HotJobs (hotjobs.yahoo.com), Yahoo Messenger, Search, News, Maps, etc.
Interesting side note: according to Alexa, almost half (49%) of Yahoo page views come from Y! Mail. Which possibly suggests that the rest of their properties aren't quite as dominant as first appears.