The 2008 Olympic Summer Games are well under way in Beijing, while the contest continues online for audience reach and engagement.

NBC spent $900 million for exclusive rights to broadcast the Beijing Olympics on TV and online. On August 9th, the day after the Opening Ceremony, nearly 2 million unique visitors dropped by NBCOlympics.com to catch the replay.

A week later, NBCOlympics.com shows no sign of slowing down, but Yahoo!’s Olympic coverage has taken the lead in US average daily reach.

Yahoo had 1.5 million average daily visitors compared to NBC’s 1.2. But where Yahoo! is winning the gold in reach, NBC narrowly leads in engagement. Of the nearly 17 million hours spent at the top Olympics sites in the US, NBC edged out Yahoo! by 5%.

Interestingly, visitors to NBC have stayed steadily longer, while length of stay at Yahoo! has been more volatile.

The reason? NBC provides exclusive access to full-length episodes of the events, even if the marquee ones are tape-delayed, whereas Yahoo! hosts short highlights and newswire stories.

As more of the 3,000 hours of Olympic event video that NBC will capture comes online, visitors have stayed longer. Meanwhile, Yahoo!’s spikes correspond to the major news events: the Opening Ceremony and August 16th, the day Michael Phelps won his 8th gold medal.