If Facebook had few investors and not much equity sold, they would have the option of offering at a cheaper price to the public -- fewer folks to be paid back. The GP comment is also suggesting they might IPO earlier, when more of their value is potential instead of actual, which also makes it cheaper and more profitable (but riskier.)

By having more private equity (or VC) rounds before IPO, more of the risk-to-money conversion happens before random members of the public can participate by buying shares.