A US security researcher has found that simply sending a web address to a friend using Facebook's private messaging function adds two likes to the page.

Reportedly, leaving a comment on a story within a Facebook page also adds to the tally.

"We did recently find a bug with our social plug-ins where at times the count for the Share or Like goes up by two, and we are working on fix to solve the issue now," the site said.

Facebook explained that the figure represented how many times an item was shared - not how often users had clicked "like" on the page.

However, the number appears next to the word "like" and the site's distinctive thumbs up icon.

This means that users who may be sharing pages to highlight negative content - such as campaigners – are inadvertently making the page appear more popular.