Believe it or not, more Americans are now members of popular social networking site 'Facebook' than have passports, statistics have revealed.

A staggering 155 million Americans -- 50 per cent of the population -- are on 'Facebook' while only 115 million people -- 37 per cent -- have a passport, according to travel website 'Tripl', which compiled the figures.

In fact, the number of U.S. citizens applying for passports has been rising over the last decade, but still the proportion of people with the travel documents is dwarfed by other countries, the 'Daily Mail' online reported.

Facebook, which was started by Mark Zuckerberg while he was a student at Harvard, has experienced phenomenal growth since it was launched in February 2004. There are now 750 million Facebook users worldwide, according to travel website Tripl, which compiled the figures.

Reasons given for the low proportion of Americans with passports include the costs of travelling abroad and the popularity of vacationing at home, according to the website. Simon Winchester, who wrote 'The Best American Travel Writing', said that Americans to do have a history of international travel.

"There was essentially no empire and hence little by way of imperial legacy. The country is formidably isolated by thousands of miles of ocean from almost anywhere truly foreign, and getting abroad is very much more costly.

"Americans seldom went to seek their fortunes overseas, as British so often did... (and there) is little tradition of American exploration," he said.

Some 13.8 million Americans were issued with passports last year, which is significantly higher than 10 years ago when 7.2 million were issued. It is a fall from 2007 though, when 18.6 million of travel documents were issued.