500m shares were traded on the first day of the Facebook IPO

"The Facebook IPO opened hot and heavy in trading on its first day, with an opening price of $38, a spike to $42 and a closing price of $38.37. According to CNBC, it also became the first IPO to see 500,000,000 shares change hands in the first day of trading.
Just 30 seconds after trading, some 82 million shares had changed hands, spiking to 100 million in just 5 minutes."

Source:  The Next Web, 18th May 2012