Ceglia says he entered into a contract with Facebook founder Mark Zuckerberg in 2003 to build the site, and that in that contract he was promised $1,000 and a 50% stake in the company. He says the contract included a clause that entitled him to a further 1% of the company per day until the site launched, which happened 34 days later--hence the 84% claim.
The suit was filed in the Supreme Court of Allegany County in New York, in which Judge Thomas Brown issued a restraining order prohibiting Facebook's assets from being moved around in response to Facebook's request for dismissal.
Facebook, in addition to requesting the case be dismissed, says the charge is "completely frivolous" and that the massive company will "fight it vigorously."
It's probably worth noting that Ceglia is actually involved in an ongoing case, brought upon his company by the New York Attorney General, Andrew Cuomo. That case accuses Ceglia's company, Allegany Pellets, of accepting $200,000 in orders for wooden pellets and neglecting to deliver either product or refund.