Some of Facebook’s earliest backers plan to sell an additional $3bn worth of shares in Thursday’s initial public offering, as they take advantage of a wave of public interest in the social network group’s flotation.

The move will increase the number of shares on offer by 25 per cent, with proceeds flowing to existing investors such as Goldman Sachs, Tiger Global Management and Accel Partners.

Investor demand has been stoked by a cross-country US roadshow over the past two weeks, allowing Facebook to increase the size of its offering from $12bn to as much as $18bn.

However, the company continues to rankle some large institutional investors who fear that the IPO’s rising price and heavy insider selling could make Facebook shares more volatile after their market debut. The company lifted its price range to $34 to $38 this week, from the $28 to $35 initially indicated.

“We’re seeing a whole new level of insider selling. It went from something you can explain away [to potential investors] to something that’s harder to,” said Sam Hamadeh, a former banker and chief executive of PrivCo, a boutique research firm.

Typical IPOs see limited selling by management and early investors, as incoming shareholders expect the interests of key board members to remain aligned with their own.

The market for Facebook’s shares is “massively oversubscribed” with especially strong demand by individual investors, according to a survey of investors by IPO Boutique, an advisory firm.

The deal now could raise as much as $18bn if, as expected, an overallotment is exercised, making it the second-largest US IPO ever.

The largest extra sales are by Tiger Global Management, a hedge fund that plans to sell 23m shares, up from more than 3m previously.

Goldman Sachs, which attracted attention for scuttling plans to market pre-IPO Facebook shares to US investors, has more than doubled the number it plans to sell, raising up to $1bn. The bank itself is selling shares as are its investment management units.

While the number of shares being sold directly by Facebook remains at 180m, worth $6.8bn at the top of the price range, board members are also selling more. Peter Thiel and James Breyer, who represents venture capital firm Accel Partners, are both increasing their sales.

Mark Zuckerberg, the company’s founder, is not selling additional shares, though the sales of shares by board members who have pledged control of their voting shares to him will reduce his voting stake to 55.5 per cent from 57 per cent, according to figures from PrivCo.