Yesterday morning Citi analyst Mark Mahaney put out a research note with comScore data that showed Facebook had passed Google in terms of time spent online. Mahaney observed:

Looking at % of Total Time Spent by the top 5 sites in August, Facebook, for the first time took the top spot with 41.1B minutes followed by Google with 39.8B minutes. Yahoo! fell to the 3rd spot with 37.7B minutes. Also, Yahoo!’s share of time spent in Q3 QTD slid to an all-time low of 9.3%. On a separate note, Google’s management noted that its number of global users crossed the 1B mark for the first time.

Mediapost’s article, largely based on the note, also points out that “Yahoo edged out Google in August to become the top site in monthly traffic, with 179 million unique visitors. Google had 178.8 million, followed by Microsoft with 165.3 million.”

Hitwise has been reporting that Facebook is the top US site for several weeks at least:

What do we make of all this? Should we care, should the involved companies?

Yahoo should probably care about the declining time spent trend that Mahaney points out above because it’s a content site and relies on user engagement as part of its overall pitch to advertisers. However the time spent figures (unless they dramatically change) shouldn’t worry Google. In fact Google Instant may diminish overall time spent, if it makes searching more efficient as promised.

But perhaps Google does care about these numbers. As Danny Sullivan pointed out to me in an email, the “1 billion user” number that Google disclosed during its Instant press conference was the first time the company has issued a number along these lines. It may have been in anticipation of coverage that would come from the comScore data showing that Facebook had passed it.

However, just as with search traffic share, trends are more important than isolated metrics. The fact of who’s number one and number two is largely symbolic and about bragging rights — for the time being.

Related Topics: Facebook | Google: Business Issues | Stats: comScore | Stats: Hitwise | Stats: Size

About The Author: Greg Sterling is a Contributing Editor at Search Engine Land. He writes a personal blog Screenwerk, about SoLoMo issues and connecting the dots between online and offline. He also posts at Internet2Go, which is focused on the mobile Internet. Follow him @gsterling. See more articles by Greg Sterling