Google passed Yahoo in its share of monthly visitors in the United States for the first time this April, buoyed by growth in search and YouTube videos, according to ComScore statistics released Thursday.
However, underscoring the variability of this sort of measurement, which extrapolates overall data from the usage of a "panel" of users at home and work, ComScore rival Nielsen Online released its own data as well with some different results. Although it also showed Google as No. 1 in terms of unique users, it said Google passed Yahoo way back in January 2007.
ComScore said Google sites had 141.1 million unique visitors in April, a tad ahead of Yahoo's 140.6 million. Microsoft was in third at 121.2 million, with AOL at 111.3 million.
Nielsen's data showed Google at 128.2 million, Microsoft at 122.1 million, and Yahoo at 117.1 million.
Nielsen also provides information on time spent at the sites, though. There, Yahoo leads its rivals with 3 hours and 9 minutes per month, but AOL owner Time Warner leads Yahoo at 3 hours 40 minutes per month.
Microsoft's usage was 2 hours and 17 minutes, and Google was 1 hour and 47 minutes, Nielsen said.