For the third month in a row, more Americans visited Yahoo's websites than Google's, according to comScore's Internet traffic data for September. The research firm on Monday reported that Yahoo's websites had 197.8 million unique U.S. visitors last month, while Google's had 191.4 million. Yahoo was ahead of Google in July and August, too.

But Yahoo has yet to transform this large web traffic to ad revenue, as it lags behind Google and Facebook.