Despite having achieved mainstream status a few years ago, the total audience for online video continues to balloon. And YouTube’s dominance in the category seems boundless, as the site delivered more than 10 times as many video streams as any other site in the U.S. last month.

According to the latest report issued by Nielsen Online, 137.4 million Americans watched Web video in December, a healthy increase of 10.3 percent versus the same month in 2008. Those viewers streamed over 10.7 billion videos during the month, representing an increase of 11.8 percent versus the same time period a year earlier.

While the number of streams per visitor showed only marginal growth, Web video viewers are watching longer clips on average; time spent (per viewer watching online video) jumped 13.2 percent to 193.2 minutes in December.

And while Hulu, the joint venture between News Corp., NBC Universal and Disney, continues to demonstrate tremendous growth—making it the No. 2 video site on the Web—YouTube continues to account for a disproportionate amount of the video consumed on the Internet. The Google-owned property streamed over 6.4 billion clips in December, per Nielsen, while Hulu streamed almost 635 million videos. YouTube also reached nearly 106 million unique viewers versus Hulu’s 13.6 billion.

Curiously, Nielsen and rival comScore continue to report audience numbers for Hulu that are miles apart in scope. While comScore’s data places YouTube far ahead of other players in the segment, it estimates Hulu’s audience to be over 43 million users—roughly 30 million users more than Nielsen’s estimates. Similarly, according to Hulu, comScore’s data indicates that the site’s average monthly streams recently topped 920 million—almost 300 million more than Nielsen tracked.