Facebook has admitted that as many as 5% to 6% of the accounts on its social network are fictitious or duplicates. That means more than 40 million of its 845m monthly active users are fake. Facebook gave the details yesterday as it updated Wall Street and filed an amendment to its IPO and added a further 25 banking firms to its roster of underwriters, including Citigroup, and Deutsche Bank, according to the FT .

It highlights one of the growing issues for all social media firms and that is of spam, which afflicts both Facebook and Twitter where at times it can see rife. Yesterday was a case in point as spammers flooded the FT’s media conference hashtag driving some users away.

For Facebook  it is an admission of serious issue as its revenue model is based around the fact that its users are real people as it forces users to sign up with their real names.

However, tens of millions are circumventing that. If 5% to 6% is the official acknowledged figure there is a risk that it could be higher.

“If advertisers, developers or investors do not perceive our user metrics to be accurate representations of our user base … our reputation may be harmed and advertisers and developers may be less willing to allocate their budgets or resources to Facebook,” the company said in the revised filing.

Facebook also made reference to its patent fight with Yahoo, in which the struggling internet company has alleged that Facebook’s technologies infringe on the intellectual property of 13 of its patents. Facebook said it was still investigating the matter.

Facebook’s filing also confirmed that the company had opened a new line of credit, totalling $8bn, largely to cover tax obligations associated with employees’ restricted stock units, the FT reports.

As an aside there was a good deal of discussion about Twitter fakes yesterday at the FT Media conference. This snippet of Twitter chat below from @suellewellyn gives a good snapshot of the spam discussion.

It suggests that tens of thousands of followers of the likes of celebrities such as Lady Gaga, who recently passed the 20 million Twitter followers mark,could in fact be fakes and spammers: