The rate of U.S. ad spending seems to be slowing down, and at fault may be internet advertising - which is, seemingly incongruously, the fastest-growing medium.

U.S. advertisers spent $7.2 billion on online advertising in the first three quarters of 2006 - more than a billion dollars more than the $6.1 billion spent in the same period in 2005, according to estimates released Tuesday by ad tracking firm TNS Media Intelligence, MediaPost reports. Those numbers don't include online search advertising.

TNS MI estimates that online display ad budgets are growing at an 18 percent rate, but search is growing at more than 30 percent a year according to TNS MI President-CEO Steven Fredericks. That growth, however, is being fueled by midsize and smaller advertisers.

The top 50 advertisers remain focused on traditional media, and although they are upping their online ad budgets… those online dollars are going further than their traditional media dollars - and the savings are being invested in traditional media, Fredericks said.