ComScore's figures show Facebook has grown at a rate of 162% from 47.2m visitors in May 2007 to 123.9m in May 2008. In contrast, MySpace has grown just 5% from 109.5m to 114.6m.
In April Facebook was marginally ahead of its News Corporation-owned rival, by around 0.5m users, but now a 7.2m gap has opened up between them.
The two sites are leagues ahead of the third-ranked Flickr.com, the photo and video sharing site owned by Yahoo!, which has 54m visitors. Its numbers have grown by 101% since last May when it had 26.8m.
The results provide a fillip for Facebook, from which some members of the original executive team have departed in recent weeks. Adam D'Angelo, chief technology officer, left in May and Matt Cohler, Facebook's first official hire and product development head, left last week.
Facebook is a private company and does not disclose official sales or profit figures. However people close to the company have claimed that it made $150m in sales last year with the figure expected to rise to up to $350m this year, according to the Financial Times.