The leading site is News Corporation-owned MySpace, which attracted more than 114m global visitors aged 15 and older in June 2007, a 72% increase on 12 months earlier.
In second place, Facebook experienced even stronger growth, jumping 270% to 52.2m visitors. Further dramatic increases were seen by Bebo, which was up 172% to 18.2m visitors, and Tagged, which was up 774% to 13.2m visitors.
Bob Ivins, executive vice-president of international markets at ComScore, said: "During the past year, social networking has really taken off globally. It would appear that social networking is not a fad but rather an activity that is being woven into the very fabric of the global internet."
The study also reveals that specific social networks tend to vary in popularity in different regions.
MySpace and Facebook attracted roughly two thirds of their respective audiences from North America. Meanwhile 63% of Bebo's visitors are from Europe, while Orkut's user base is predominantly in Latin America and Asia Pacific.
Ivins added: "A fundamental aspect of the success of social networking sites is cultural relevance. As social networking continues to evolve it will be exciting to see if networks are able to cross cultural barriers and bring people from different corners of the globe together."