The event, which featured Donald Trump in his first debate, was also the highest rated telecast in the nearly 20 year history of the Fox News Channel, a spokeswoman said.  Of the 24 million viewers, 7.9 million were in the key advertising demographic of 25 to 54 year olds.  The ratings, which were delivered by Nielsen on Friday afternoon, shattered even the rosiest expectations for the beginning of debate season.  The numbers from Thursday night's two hour debate tripled the largest audience for any previous debate.  For perspective, the debate's audience easily exceeded the David Letterman farewell show (13.7 million viewers) and virtually tied the most watched "Sunday Night Football" game last season, the Dallas Cowboys versus Philadelphia Eagles game which drew 24.3 million viewers.  Trump is a known ratings magnet. His reality show "The Celebrity Apprentice" used to reach 20 million viewers a week, but it has slipped over the years, averaging 6 to 8 million viewers for recent seasons.  Now, through the debate process, it seems, Trump's ratings are back up.  About 6.1 million viewers tuned in for the "undercard" debate at 5 p.m. That means the seven candidates who didn't make the cut for prime time, like Rick Perry, reached just a quarter as many viewers.  The ratings are good news not only for Fox but for other media organizations that are sponsoring Republican debates. The "Trump show," as some have called it, seems likely to continue into the fall.  The next debates are set for September 16 (on CNN) and October 28 (on CNBC).  Related: has Fox News turned against Donald Trump?  Fox had high expectations for Thursday's kick-off to debate season. Rival executives and campaign aides also predicted unusually high ratings for the event, thanks to curiosity about Trump and the controversy over the "top ten" candidate criteria.  But the overnight numbers seemed to surpass everyone's expectations.  There was speculation on Friday morning that it may go down in the record books as the highest-rated Fox News telecast ever.  On Thursday night there was immediate indication of the audience's interest: the debate dominated social chatter on Twitter and Facebook.  There was also trouble with Fox's web stream.  Fox made the debate available online to cable and satellite subscribers, but many users on Twitter said they had a tough time accessing it.  A Fox spokeswoman confirmed the problems and said "an unprecedented, overwhelming demand caused server issues in the debate live stream."  The Future of Media, a customized magazine  CNNMoney (New York) August 7, 2015: 2:26 PM ET
Fox's GOP debate was watched by 24 million viewers on Thursday night, according to Nielsen data, making it the highest-rated primary debate in television history.