Television is losing ground in one of its most lucrative strongholds: Thursday nights. Fewer people in the U.S. are watching Thursday prime-time TV so far this fall, as changing viewing habits, weaker shows and more varied competition take a toll on a night that for decades has showcased some of TV's most high-profile programs, and priciest commercials. Over the first four Thursday nights of the TV season, an average of 48.5 million people between the ages of 18 and 49 years old were watching prime-time on any channel, broadcast or cable, recorded or live, according to Nielsen Co. estimates. That...