Here’s an interesting milestone that YouTube has just achieved: In May 2010, it surpassed the 100-video mark in the average number of videos shown monthly to its U.S. viewers.

Data released by comScore, a marketing research company that collects data for many of the Internet’s largest businesses, show that ~183 million people watched online videos during May. (By the way, that’s nearly 85% of the entire U.S. Internet audience.)

With YouTube accounting for ~14.6 BILLION videos served, it translates into 101 videos for the average viewer. The duration of the average online video shown was a little over four minutes.

How pervasive is YouTube? The May comScore stats show that it accounted for far more activity than any other video site, charting ~43% of all videos viewed. Hulu ranked second, with the various Microsoft video sites ranking third.

And the contest isn’t even close: Hulu’s second-place ranking was good for only ~4% of viewership!

The average number of videos seen monthly per viewer as recorded by comScore were as follows:

 YouTube: 101 average number of videos per viewer
 Hulu: 27
 Microsoft video sites: 16
 Viacom Digital: 10

If there were any continuing questions as to who is the 500-pound gorilla in online video, these statistics appear to be putting that debate to rest.