Facebook's owners now plan to sell 25 per cent more shares in the company as investors clamour for shares in the year's hottest stock offering.   

Facebook says in a regulatory filing that it will now sell about 421 million shares. That's up from 337 million previously. That doesn't include more than 60 million additional shares that could be sold to cover excess demand.   

If all the shares are sold at the high end of the expected price range, the offering could total more than $18 billion. That would make Facebook one of the largest IPOs ever.   

The increased size signifies heavy investor demand for the shares. It comes a day after Facebook raised the price range it expects for the stock.   

The offering is expected to set the final price Thursday evening. Shares would start trading Friday.