Web analytics company comScore this week released its top fifty US Web properties list for May 2008 based upon its Media Metrix service, and Google’s leading the pack by a nose.

In April, comScore reported a surge in Google queries, taking it up a whole 1.8% from March, when all other search engines dropped. Strictly as a Web property, however, Yahoo is usually comScore’s favorite. It enjoyed several months on the top of the hot properties list, only now to be taken down a notch by Google.

 The difference between Google’s 75% of the total US audience and Yahoo’s 74% was only 464,000 visits. This period of Google dominance has been punctuated by frequent news of Yahoo tumult. In the month of May alone, Yahoo lost Director Ed Kozel, faced criticism for problems with the beta of McAfee-powered Yahoo SearchScan, and of course, dismissed the well-publicized Microsoft bid and saw subsequent dissidence from investor Carl Icahn.

During the month of May, Google properties expanded with the public release of Google Sites, the beta launch of Google Health, and an update to Google Docs.

In terms of the Web at large, comScore cites the Mother’s Day holiday as a major driver of traffic. Proflowers.com showed a 319% month-over-month explosion, and the whole category of Flowers, Gifts and Greetings appreciated a 15% spike in unique visits.

Memorial Day, the company says, teamed with domestic economic conditions, made coupon and travel sites surge 11% and 9%, respectively, as well.