They sued Mark Zuckerberg for $65m. But it was not enough

A court settlement of $65m (£41m) would put an end to most disputes, but not when it stacks two of the most aggrieved men in corporate history against their sworn enemy, the billionaire founder of Facebook.

Tyler and Cameron Winklevoss, the twin brothers who accused Mark Zuckerberg, 26, of stealing their idea for a social networking site, have restarted the acrimonious dispute (*See below) after filing a new legal claim against their former friend.

The 29-year-old identical twins, who were cruelly labelled the "Winklevii" in this year's partly fictional account of Facebook's founding, The Social Network, allege they were misled over the true value of the site when they launched legal proceedings in 2006.

Facebook responded to the suit with legal papers of its own and accused the brothers of "suffering a bout of settler's remorse". In documents filed at the United States District Court for the Northern District of California, Facebook said that the brothers had calculated the value of the company using a press release issued by Facebook several months before the settlement – and that it was under no obligation to submit alternative figures.

The papers say the deal was "enforceable because it clearly communicated the parties' intention to be bound and the terms were definite", adding that "their fraud claim is based on omission: they fault Facebook for not volunteering a more recent – and, they claim, lower – valuation of different Facebook stock. They insist that their sworn enemy had some special duty to open its books and volunteer any information that bears on the value of this closely held company".

The bitter dispute began in 2003 while the twins and Zuckerberg were students at Harvard University. The twins allege they, along with fellow student Divya Narendra, had a verbal agreement with Mr Zuckerberg to build the programming for their Harvard Connections site. Months later, Mr Zuckerberg created the rival site that eventually became Facebook, which now boasts over 500 million users. Mr Narendra, who also took a settlement payout in 2008, is not a co-claimant in the Winklevoss twins' most recent claim.

At 6ft 5in tall, the twins cut striking figures. Born into a wealthy New York family, the high-achieving brothers majored in economics at university before getting MBAs at Oxford. Both have rowed for Oxford and the US at the Beijing Olympics.

But their perceived betrayals by Mr Zuckerberg, which has banked him about $6.9bn and makes him the 35th richest man in America, has clearly left them feeling like they have lost out.

This would not be the first case of "settler's remorse". Patricia Cohen, the former wife of Wall Street tycoon Steve Cohen, the 32nd richest man in the US, lodged a $300m suit against him last year after claiming he had hidden assets and bank accounts during divorce proceedings in the 1980s.

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*A statement from Facebook says:

"There is no new litigation between Facebook and the Winklevosses. The only recent litigation activity is when the Winklevosses recently lost their malpractice action against their former law firm, Quinn Emmanuel. The filings referred to here are simply the filings by Facebook and the Winklevosses in the Winklevosses' now two-year old, thus far unsuccessful, attempt to undo their 2008 agreement to settle the parties' dispute."