The stock price is down about 9.8% to $28.80 in afternoon trading — well below the May 18 initial offering price of $38. One likely reason for latest decline is that beginning today investors who are interested in Facebook, but don’t want to take the risk of buying the shares, can buy options in the stock. There’s been heavy trading on these  bets that enable customers to buy or sell Facebook shares at a pre-set price on a specific date. Also weighing on the stock is the rumor, which began to circulate over the weekend, that Facebook is angling to buy Norway’s Opera Software, which makes Web browsers for mobile devices. The social network company has been snake bit since it went public, and especially following disclosures that the underwriters of its IPO — Morgan Stanley and Goldman Sachs– told institutional investors, but not the public, that they had lowered revenue estimates for Facebook.