Facebook shares, which struggled to stay above their $38 IPO price following the launch of trading on Friday, have come under selling pressure in pre-market trading on Monday. With a little more than 30 minutes to go before the opening bell, Facebook shares are down $1.25, or 3.3%, to $36.98. There are ongoing concerns that the company and its bankers simply stretched the valuation further than they should have. After initially setting planning to sell stock at $28-$35 a share, the company raised the target price range to $34-$38, and then expanded the size of the offering by 25% to significantly increase the number of shares sold by insiders. The deal then priced at the top of the range, traded up briefly to around $42, but then struggled to take flight amid widespread reports of trading glitches on Nasdaq. Shares of other social networking sector players, including LinkedIn, Groupon and Zynga, are all likewise trading lower in the pre-market session.