SAN FRANCISCO (MarketWatch) -- Shares of Facebook Inc. fell more than 8% to $35.09 on Monday morning, putting the stock below its IPO price of $38 per share on its second day of trading. Facebook FB, +1.93% made its trading debut on Friday following its $16 billion IPO, with the shares closing the day with a gain of only 0.6%. More than 35 million shares of the social network were traded at least check.