Facebook.com accounted for 25 percent of page views in the United States for one week in November, up from 16.7 percent this time a year ago.
The social network, which grew 60 percent from a year ago, also logged 1 in 10 U.S. Internet visits, according to researcher HitWise. Google notched 7 percent of visits.
The page view traffic, almost four times that of No. 2 destination YouTube and five times that of Google, isn't a big deal on its face. It's the other stats' ramifications for the online ad market that are huge.
Facebook also leads in online engagement time. comScore said in September U.S. Web users in August spent 41.1 million minutes on Facebook, compared with 39.8 million minutes on all of Google's Websites.
Google for years has been the top destination in traffic, page views and minutes spent online, buoyed by its own search engine, Gmail and the popular YouTube video-sharing Website.
Facebook's rise may make it the top target for advertisers seeking a larger online audience with the social network's 500 million-plus users.
Worse still for Google is that Facebook Connect and the social plug-ins such as the Like button help the social network extend its tendrils outside the Facebook.com walled garden.
So even though Facebook has no "FaceSense," or third-party ad platform to challenge Google's AdSense, and Facebook CEO Mark Zuckerberg gave no indication one was coming at the Web 2.0 Summit last week, the brand saturation is there.
This is a big threat to Google, which is planning to counter Facebook by adding social layers across its properties.
Google has already taken several steps to address the local search and ad market, launching Google Places, Tags ad call-outs, the Boost SMB ad platform, Place Search and the Hotpot local recommendation engine.
However, there is no social glue connecting those Websites to keep users visiting them for anything other than fleeting directional lookups and business ratings and reviews.
That's a problem Google must remedy if it is to keep users coming back.