Yahoo’s (YHOO) U.S. audience was smaller than Google’s (GOOG) by about 6 million people in the month of August, but Yahoo visitors lingered about twice as long, according to data released today by research firm Nielsen//NetRatings.

Microsoft (MSFT) actually had the largest U.S. online audience according to the firm, which counted all of the company’s Web properties including its MSN and Windows Live sites. Google was second and Yahoo third, followed by Time Warner (TWX), News Corp. (NWS), eBay (EBAY), InterActiveCorp (IACI), Amazon (AMZN), Walt Disney Internet Group (DIS), and Wikimedia Foundation. (Time Warner is the parent company of Fortune.)

Though they were not at the top of the list in terms of audience, visitors to Yahoo and Time Warner sites stayed significantly longer than visitors to most of the other sites.

Nielsen also included a list of the top online advertisers with estimated spending for August, which included Low Rate Source, NexTag, Experian Group (EXPN), IAC, Countrywide (CFC), AT&T (T), NetFlix (NFLX), Verizon (VZ), Monster Worldwide (MNST), and Privacy Matters.