Eight years after its launch, social networking giant Facebook is poised to file papers for an initial public offering seeking to raise at least $5 billion.

The prospectus for an IPO could be filed with the US Securities and Exchange Commission (SEC) as early as Wednesday (US standard time) morning, according to US media reports, although they stressed there was still a possibility it could be delayed.

The Wall Street Journal broke the news of Facebook's impending IPO last week, saying the Palo Alto, California-based company planned to raise $10 billion at a valuation of between $75 billion and $100 billion.

The $10 billion target had been halved to a more conservative $5 billion in reports emerging late Tuesday in The New York Times and International Financing Review.

The publications stressed, however, that $5 billion was only a preliminary fund-raising target and could always be raised after gauging investor interest.

The initial IPO filing was not expected to specify how many shares were being offered or their list price.

Facebook shares are not expected to begin trading until at least May, once the IPO process is finalised.

With a deal size of $10 billion. Facebook would slip into sixth place on the list of largest US IPOs between AT&T Wireless Group ($10.62 billion) and Kraft Foods ($8.68 billion), according to Renaissance Capital.