The increase has United, English champions a record 19 times, comfortably ahead of the world's second-most valuable sports team, the National Football League's Dallas Cowboys, worth $2.1 billion.
Forbes put the surge in United shares down to brighter earnings prospects from new sponsorship deals and said the demand could continue given the team's potential for lucrative payouts in the EPL and Champions League.
United, who claim to have 659 million followers worldwide, are owned by the American Glazer family who retained a tight grip on the club after the flotation on the New York Stock Exchange. United shares closed 41 cents lower at $16.48 in New York on Monday.
Becks training with Arsenal
Former England midfielder David Beckham has been training with Arsenal, the club's manager Arsene Wenger has confirmed, but rejected reports that could sign. “He’s asked to come here and to work on his fitness. He has not done anything for a long, long time," said Wenger.