Facebook says it will sell 25% more shares than first planned in its flotation in response to strong demand.
The move comes one day after the social networking giant said it would raise the price of the shares by 21% to between $34-$38 a share.
It also comes despite doubts about the profitability of the site, which is largely used for social updates.
Car giant General Motors added to those doubts by saying on Tuesday it would no longer pay to advertise on the site.
However, rival Ford said it would continue its social media strategy. A spokesman said: "You just can't buy your way into Facebook. You need to have a credible presence and be doing innovative things."
Facebook will add about 84 million shares to its initial public share offering (IPO) and will now sell about 421 million shares, up from 337 million, raising $18bn (£11.3bn).
This is still only a small percentage of the entire company, and implies Facebook's full market value is around $100bn, similar to that of internet shopping giant Amazon.