Social networking site tops in London, Toronto

"University students in that regard are big users...there are a lot of students (in Toronto) and when you add them all up, when these students link up to friends and relatives outside -- it's kind of a snowball effect."

Federman added that Toronto's strong ethnic mix may also mean a greater likelihood for people wanting to connect with others all over the world. The relative affluence of the city also means a larger percentage of the population has access to high-speed internet.

"I think that certain of the factors might equally apply to London, or any large urban area," Federman added.

Half of users of Facebook, which is a growing favorite with the 25 and up age group, make daily visits to the site.

Membership in the New York, Los Angeles and Chicago regional networks combined is about 918,168, while Washington D.C., boasts 272,723 people on its network.

Dissecting Facebook by numbers is not very straightforward. Users can join multiple networks or chose not to associate themselves with any network. But the figures are significant enough for researchers to be interested in how social communities evolve and expand.

Chan said that he is now seeing colleagues look at these types of questions as serious research, but added: "We don't have the answers yet."