The 2010 World Cup Final had an estimated TV audience of 700m

"Spanish broadcasters said Ole! to record ratings as nearly 17 million fans watched the country's one-nil victory over the Netherlands in the finals of football's World Cup, part of about 700 million viewers estimated to have caught the match on small screens worldwide.
Some 16.8 million Spaniards -- 91 percent of the TV audience -- caught the highlight of the match, when Andres Iniesta rocketed in the winning goal in extra time giving Spain its first-ever World Cup win. On average 15.6 million people in Spain, 86 percent of the audience, watched on the three channels broadcasting the match, an all-time average record, according to ratings agency Barlovento Comunicacion.
Similarly in the Netherlands, 91 percent of the TV audience tuned into public broadcaster NOS as their Orange-mantled team failed in its third World Cup final. An average of 8.5 million Dutch caught the match, with millions more watching in pubs and public squares across the country. (Spain's population is 46 million vs. 17 million for the Netherlands.)
Though final figures aren't available, soccer's governing body FIFA estimates around 700 million people watched the final live. If that is correct, the match would beat out the estimated 600 million that caught the opening ceremony at the 2008 Beijing Olympics and would be on par, or slightly above, the 700 million that watched the World Cup final four years ago."

Source:  Reuters, 13th July 2010

Note - Neilsen say that over 2 billion saw the 2008 Olympic Opening Ceremony