As measured by traffic rankings from Alexa, per January 2008. Let me know if your have updates. (Via Scampblog)

Analysts believe there’s a maturity and reliability to this ad boom that was sorely missing during the Internet bubble. In 2006, Google is expected to get nearly one-quarter of the $15.6 billion that market research firm eMarketer estimates will be spent on online advertising in the U.S., compared with the 20.7 percent Yahoo is expected to get, said David Hallerman, a senior analyst at eMarketer.By most accounts, the online-ad market barely has been tapped. Online ad spending, which represents only 5 percent of total media spending, is projected to grow 24.4 percent this year, while all media, including television, radio, billboards, newspapers and direct mail, is projected to grow only 4.2 percent, according to eMarketer.Online ad spending in the U.S. rose to a record $12.5 billion in 2005, according to a study released last month by advertising trade group Interactive Advertising Bureau and independent auditor PricewaterhouseCoopers.By 2010, online ad spending in the U.S. is expected to rise to $23.5 billion, according to market research and consulting firm Parks Associates. Worldwide, online ad spending is forecast to grow from $19.5 billion in 2005 to more than $55 billion in 2010, according to Piper Jaffray.Search is by far the most lucrative area, accounting for 40 percent of the total online ad spending in the U.S., according to JupiterResearch. Search advertising is expected to grow from $4.2 billion in 2005 to $7.5 billion in 2010, while display advertising is forecast to grow 10 percent between 2005 and 2010 to $7.5 billion, JupiterResearch has forecast.