Facebook also said on Wednesday that it had secured two new credit facilities, one of which will help the company satisfy hefty tax withholding obligations and remittances related to employees' stock units following its initial public offering.

Facebook, the world's largest Internet social network, with more than 845 million users, is preparing to raise $5 billion in an IPO that could value the company at up to $100 billion.

The company, which generated $3.7 billion in revenue in 2011, is increasingly challenging established Internet giants such as Google and Yahoo for users' online time and for advertising dollars.

In an amended prospectus filed with regulators on Wednesday, Facebook acknowledged that 5 percent to 6 percent of its 845 million monthly active users are "false or duplicate accounts" according to its estimates. Facebook also said it estimated that less than 5 percent of its daily active users may be the result of mobile applications which automatically contact the company's servers with no user action involved.

Facebook also cited a brewing legal dispute with Yahoo, noting that Yahoo sent the company a letter last month alleging that "a number of our products infringe the claims of 13 of Yahoo's patents." While Yahoo has not commenced any legal action, Facebook noted that Yahoo may do so in the future.

The new banks, including Citigroup, Credit Suisse and Deutsche Bank, increase the number of underwriters on the deal to 31, the filing said.

Among the two dozen new banks getting a piece of the Facebook IPO were several smaller firms, including Oppenheimer & Co, Pacific Crest Securities and Cowen and Co.

"The more banks you have, the more coverage you will have from those firms on the research side in the future. These banks will get paid well and there's a lot of prestige associated with the Facebook IPO," said Dan Niles, of the hedge fund AlphaOne Capital Partners.

Inclusion of smaller financial firms also could help ensure a wide distribution of Facebook's stock, he said.

"Small firms will typically have some retail accounts, so it's a way to spread shares out to those people who may not have access to Facebook stock," said Niles.

Facebook disclosed a new $5 billion credit facility for "general corporate purposes," replacing a previous $2.5 billion credit line.

The company also entered into a bridge credit facility, allowing it to borrow up to $3 billion to fund tax withholding and remittance obligations related to the settlement of restricted stock units, the filing showed.

(Reporting By Alistair Barr; Editing by Carol Bishopric and Steve Orlofsky)

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