There is something apt about a social networking website winning a popularity contest. According to industry data, Facebook overtook Google among US internet users last week, with more visits to its pages than to the search engine. It is a moment to consider the rapid growth of a site whose 400m-plus users outnumber the population of any single country except India and China.

The industry data come with a few caveats. The figures exclude visits to other Google services, such as YouTube and Google Mail. They omit searches carried out in a box on a browser toolbar. Also, the number of visits is just one measure of internet take-up: counting unique users – visitors rather than visits – gives a different profile. Still, it highlights the momentum behind Facebook as it displaces Google.com from the weekly lead it has commanded on this measure since September 2007.

Advertisers find Facebook appealing too. It enables them to reach a mass audience, as television does, but with the extra benefit of much greater targeting. Consumer brands could easily extend their presence beyond the fan pages that already exist. Moreover, a social site provides consumers who visit for much longer than they would use a search engine. So increased advertising, and perhaps ways to allow users to shop through the site, should enable Facebook to move from positive cash flow to making profits.

It will need to tread carefully. There is a risk for advertisers – and for the site – if Facebook moves to become commercial in a way that users resent. In amongst personal information, advertisements are more likely to strike a jarring note.

The high-growth phase means that Facebook can take its time developing ways to increase revenues. The key must be to find ways that bring practical benefits to those who visit the site. There is an intrinsic stickiness about a site where users have assembled their own material, but if people stop updating their pages and social networking takes a new form, then winning users back is a hard task.

What the data do not show is that search engines have had their day. Google's core search advertising business rebounded in the final quarter of 2009, and the group is preparing for renewed growth. Moreover, there is a straightforward reminder of how fragile the fortunes of social networking sites can be: the site that Google overtook in 2007 to become most popular in the US was MySpace – which is now seeking a new role as social users have moved elsewhere.