comScore has released its online video rankings for sites in the United States for May 2012 and it appears to be some more good news for Yahoo. Following last week’s comScore report that Yahoo had finally slowed the precipitous loss of its search market stakes in May, this month’s video site rankings show that Yahoo appears to have gained back in May some of the viewers that it lost in April. The company’s total unique viewers still isn’t back up to March’s 60 million viewers, but at least Yahoo appears to be headed in that direction.

Google sites, which include the very popular YouTube, lost about 6 million viewers in May, which may or may not have something to do with Yahoo’s climb back up towards its March numbers. Then again, one reason Yahoo made some significant gains in May may have something to do with the fact that it was offering over 100,000,000 more videos in May than it was in April.

In other changes from April, Microsoft was bumped down one place by Facebook, Viacom was bumped down one place by AOL, and News Distribution Network bumped down both Amazon and Hulu.

The top five online video ad properties was unchanged from April to May and the second five shifted around very little. Hulu remained the top video ad property by adding about 69 million ads in May, amounting to about 55 million ad minutes. Hulu also delivered the highest frequency of video ads to its viewers with an average of 56. In other words, Hulu’s really trying to give you the television experience with lots of ads.

Also of note, comScore says that video ads accounted for 21.6% of all videos viewed and 1.9% of all minutes spent watching online video. While the average online video lasted about 6:30, the average video ad lasted about 12 seconds. The amount of video ads posted in May yielded another new record, surpassing the 10 billion mark for the first time.

VEVO continues to dominate the rankings among YouTube Partner Channels. Although it added nearly 27 million new videos in May, it actually lost over 600,000 unique viewers. Excepting the final two spots in the top ten YouTube channels, the rankings for May remained the same.

Although there were a few variations in May’s rankings from April’s, comScore notes that the number of the U.S. audience for online video remained the same at 84.5%.