Online advertising revenues exceeded $21 billion for the first time in 2007, although preliminary data compiled by an industry trade group also suggest growth is slowing. The Interactive Advertising Bureau said its estimates show ad revenues grew 25 percent last year from nearly $17 billion in 2006. In dollar amounts, the estimated gain was $4.2 billion - less than the 35 percent and $4.3 billion growth seen in 2006 over 2005.

Analysts have said the growth rate was bound to slow as the Internet commands a larger share of the advertising pie, taking dollars away from traditional media like newspapers. By most accounts, the Internet still represents less than 10 percent of all U.S. ad spending, meaning there's room for a lot more growth, even at a slower rate.

PricewaterhouseCoopers LLP, which conducts a quarterly survey for the advertising trade group using data from the 15 largest online ad sellers, said fourth-quarter revenues totaled about $5.9 billion, topping the previous record of $5.2 billion in the third quarter.