Facebook has admitted that more than 83 million accounts on its social network may be fake, representing 8.7 percent of its 955 million active users.

In documents filed with the Securities and Exchange Commission (SEC) earlier this week, Facebook divided fake profiles into three categories: “duplicate”, user-misclassified” and “undesirable”.

A duplicate is “an account that a user maintains in addition to his or her principal account,” user-misclassified accounts are those created for “a business, organisation, or non-human entity such as a pet,” and undesirable accounts are those deemed to be in breach of Facebook's terms of service.

Facebook revealed that duplicate profiles make up 4.8 percent of the users, user-misclassified accounts account for 2.4 percent, and 1.5 percent of users were undesirable. Numbers are based on a sample of live accounts.

The news is significant given that the vast majority of Facebook's revenue is from advertising. Advertisers pay for ad products, including sponsored stories in users' News Feeds, based on the number of impressions delivered or the number of clicks made by those users.

“The loss of advertisers, or reduction in spending by advertisers with Facebook, could seriously harm our business,” the company said in its filing.

“Advertisers will not continue to do business with us, or they will reduce the prices they are willing to pay to advertise with us, if we do not deliver ads and other commercial content in an effective manner, or if they do not believe that their investment in advertising with us will generate a competitive return relative to other alternatives.”

An investigation by the BBC last month revealed that companies are wasting large sums of money on adverts to gain “likes” from Facebook members who have no real interest in their products.

It also found that some accounts are run by computer software with one person controlling thousands of profiles from a single desk, handing out commands such as: 'like' as many pages as you can to create a large community.

“Companies who are considering advertising on the social network want to be sure that any 'likes' they receive are from genuine users, not bogus accounts,” said Graham Cluley, senior technology consultant at Sophos, in a blog post.

“Of course, it's far from simple for Facebook to determine reliably if every account is fake or not, as anybody can create an account with a bare minimum of credentials.

“What remains to be seen is whether the proportion of dodgy accounts on Facebook continues to grow, and if the site's advertisers view it as a problem.”