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They call Facebook (NASDAQ: FB) and Alphabet’s (NASDAQ: GOOGL) Google the online duopoly. The two companies control most of America’s online revenue and the pie left for others has shrunk so fast it has started to cripple the industry. New research shows the extent of their dominance: Google and Facebook are by far the most visited web properties in the U.S., based on unique visitors

The total online advertising industry is forecast to produce $83 billion in revenue this year. emarketer reports that 40% of the market belongs to Google and 21% to Facebook.

Research firm Comscore measures online visits for both mobile devices and computers. In June, Google sites had 241 million unique visitors according to Comscore. Facebook was second at 204 million.

Yahoo! was third at 186 million unique visitors. Now owned by Verizon (NYSE: VZ) its portal stablemate AOL was seventh with 154 million. emarketer says that Yahoo! and AOL each have less than 5%. In the rapidly growing mobile market, the lead Facebook and Google have grows.

Microsoft (NASDAQ: MSFT) ranks 4th in terms of monthly unique visitors. This includes its MSN portal which competes with Yahoo! and AOL. It had 183 million unique visitors in June.

Among the other notable information in the study, e-commerce monster Amazon (NASDAQ: AMZN) ranked 5th with 180 million unique visitors in the month. Walmart (NYSE: WMT) was not as far behind as many observers might suspect, based on Amazon’s dominance, with 89 million unique visitors in June. The Achilles Heel of the bricks and mortar industry was on display in the Comscore study: No other retailer was among the top 50 sites in June based on unique visitors.

Among other carefully followed rivalries, the New York Times properties had 85 million unique visitors, against Washington Post sites at 76 million. Each company has a paywall, so the figures are impressively large. The power of the Netflix (NASDAQ: NFLX) subscriber base pushed its unique visitor number to 77 million.