It could be due the climate, the large number of students or the ethnic mix of the people who live there, but whatever the reason London and Toronto have the highest number of Facebook users.

The social networking Web site, which began in a dorm room at Harvard University in 2004 for students, says it has more than 41 million active users. Its impressive growth has made it an Internet and social phenomenon.

According to media reports, Toronto was top of the Facebook world as late as the end of June, but London surged ahead over the summer.

Facebook said it does not keep statistics on networks, but a look through the regional areas in the top three countries -- the United States, Canada and Great Britain -- shows that London had 1,159,185 users, while Toronto was a close second with 836,605.

Experts agree that it's hard to explain why certain social communities take off and others don't, but speculate on possible factors.

Historically speaking, the harsher the climate and the more dispersed the population, the more telecommunication has had a major influence in shaping the culture, said Mark Federman, a researcher at the University of Toronto.

There has always been a greater per capita early adoption of communication technologies in Canada compared to the USA and a similar situation exists in Northern Europe, he added in an email interview.

Federman also believes the penetration of high speed Internet access in the United States, even in urban areas, is lower than in Toronto or in Europe.

Like London, Toronto is the home of various universities and about half a dozen community colleges, so there is a large population of demographically-right potential users, according to Federman.

Leslie Chan, a social science lecturer at the University of Toronto, agrees.

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.