While Google continues to dominate in search market share and volume, social media sites are the only properties showing positive growth, according to comScore's numbers comparing May to April.

For expanded search rankings, the comScore May 2009 analysis shows Google's (NASDAQ: GOOG) sites leading with 13 billion searches. Yahoo sites ranked second with 3 billion searches, followed by Microsoft sites with 1.2 billion and AOL properties trailing with 721 million.

Yet, as it turns out, the big players aren't the ones seeing the most growth in search.

Instead, it's Craigslist that leads in percent growth according to comScore. The online classifieds site posting a 12 percent jump in queries from 583 million to 651 million from April to May.

Facebook came in as the second fastest-growing site, with a jump to 184 million from 176 million --a 5 percent increase. Google-owned YouTube checked in at 4 percent, up from 3.2 billion to 3.3 billion.

In the same category, overall Google sites had no change month-to-month, while Google itself dropped 2 percent to 9.6 billion. Both Yahoo.com and Yahoo's related sites saw a 4 percent decrease, as did Microsoft sites.

Meanwhile, high-flying Amazon showed a 2 percent dip in its searches, down to 185 million queries. Rival eBay has more volume, with 654 million searches for April, but fell further, with a 3 percent decrease to 634 million for May.

While increased traffic at free online classified site Craigslist may be a case of consumers hunting for deals while strapped for cash during the recession, the news of social networking sites' continued growth comes as social media are also being eyed for a larger role in online marketing.

This is due in part to the souring economy squeezing online ad budgets but also because marketers increasingly want to create brand awareness with the millions of loyal community members at sites such as Facebook.

Google dominates

Google, however, still leads the core search sector, with its sites topping the U.S. core search market in April with 65 percent of all searches conducted, comScore said. Google was followed by Yahoo sites with 20 percent, Microsoft properties (8 percent), Ask Network (3.9 percent) and AOL (3 percent).

While that's not surprising, the percentage change from May to April shows lackluster performance: Google's and Ask Network's share percentage inched up by less than 1 percent, while the other three dipped by less than 1 percent.

The same pattern shows for the number of core searches. Americans conducted 14.3 billion searches at the largest search engines, the study found, with Google accounting for 9.3 billion core searches, followed by Yahoo with 2.9 billion and Microsoft with 1.1 billion.

However, the percentage change in core search from May to April is down overall for the category by 3 percent, and is negative across the board. Google dipped by 2 percent, Yahoo by 4 percent and Microsoft by 5 percent. Ask Network declined by 1 percent while AOL tanked by 12 percent.

The comScore data does not include Microsoft's (NASDAQ: MSFT) newly launched Bing search engine, and some industry watchers say Google may have to start taking notice of the competition. Early research from comScore in separate reports show Bing doing well out of the gate -- gaining in market share in its first two weeks.

Google may be showing a readiness to fight back. The site's home page earlier today displayed a link inviting visitors to "explore the world of Google search," leading to a page that touts a wide range of features available through Google's search interface -- many of which are similar to features touted by Microsoft's Bing.

Nevertheless, Google said it's looking forward to competing with new rival in the space.

"We welcome competition that helps deliver useful information to users and expands user choice. Having great competitors is a huge benefit to us and everyone in the search space -- it makes us all work harder, and at the end of the day our users benefit from that," a Google spokesman told InternetNews.com.

He added that Google often runs promotions to educate consumers on its features, citing a holiday promotion as an example.