Back in June, Steve Burke described what he called his Olympics "nightmare".

"We wake up someday and the ratings are down 20 percent," the chief executive officer of NBCUniversal said at a conference.

"If that happens, my prediction would be that millennials had been in a Facebook bubble or a Snapchat bubble and the Olympics have come, and they didn't know it."

He has escaped that with the Rio Games this year - but not by much. Prime-time broadcast viewership has been down about 17 percent compared with the London Games four years ago.

And in the 18-to-49-year-old age group coveted by advertisers, it's been even worse. That audience has been 25 percent smaller, according to Bloomberg Intelligence.

The Summer Olympics ratings slip, the first since 2000, raises fresh doubts about what used to be a sure thing: Live sports would be a huge and growing draw no matter what. That's why NBC parent Comcast Corp paid $12 billion for exclusive US broadcast rights to the Olympics through 2032. Others, including The Walt Disney Co's ESPN, 21st Century Fox Inc, Time Warner Inc and CBS Corp, have made long-term bets on soccer, baseball and basketball.

To be sure, many sporting events are as big as ever. The Super Bowl in February pulled in 112 million viewers, making it the third most-watched event in TV history. On the other hand, Villanova's victory over North Carolina in the men's college basketball championship drew 37 percent fewer viewers than last year's title game, though that may have had something to do with the fact that, for the first time, the matchup was on a cable channel, not a broadcast network.

One issue is that many fans are getting older. The average age over the past decade of National Football League and Major League Baseball viewers has increased by four and seven years, respectively, to 47 and 53, according to Ben Thompson, founder of the blog Stratechery.

Bloomberg