It is a historic day for The New York Times Co.—just not the good kind. The publisher's shares on Thursday had its biggest one-day drop since at least 1980—and then its stock plunged to $8.31 Friday. The surprise decline reflects falling ad sales and mounting challenges facing the newspaper industry as advertisers continue to be wary in an uncertain economy. Print-ad sales for the company during the third quarter dropped 11 percent, and overall ad revenue fell 8.9 percent—a downward trend that’s expected to continue into the fourth quarter. It looks like incoming CEO Mark Thompson is forging into rough waters.