As Lock-Up Expires, Groupon's Shares Plunge While the stock markets have been having a bad day on Friday, Groupon has been having a really bad day. Shares in the online coupon giant tumbled as much as 10 percent on Friday, and by midafternoon remained mired at about $9.65 a share. That's less than half of Groupon's offering price of $20 a share. It was such a precipitous descent that circuit breakers kicked in, limiting the ability to short Groupon shares for the rest of the day. The most likely culprit was the expiration of a lock-up period that prevented insiders from selling their holdings. The end of that prohibition meant that even more shares were free to flood into the market, weakening a stock that has already taken a pounding this year. The company has fallen more than 53 percent so far this year. This isn't unique to Groupon. Shares in Zynga tumbled 8 percent the day after a lock-up on the social-gaming company's stock expired. On Friday, Zynga shares were down 4 percent in mid-afternoon trading, at $6.01. Of course, there's another high-profile technology stock that has suffered a St. Sebastian-like attack as of late. Shares in Facebook were down more than 6 percent on Friday, at $27.75, after having fallen even lower earlier in the day. They remain 27 percent below the social network's offering price.