American web surfers continued to flock to Google in April, using the internet search site for 50% of their 5.3 billion queries, according to a research report.

That marked a rise from Google’s year-ago market share of 47%, while second-place site Yahoo held steady at 22%, and Microsoft’s MSN dropped from 12 to 11%, according to Nielsen/NetRatings.

The trend mirrors an announcement that Dell has agreed to load as default on new consumer PCs certain Google software, such as its desktop search application and browser toolbar, at the expense of competing Microsoft products.

Google cannot sit on its lead for long, warns Michael Lanz, vice present of search industry solutions for Nielsen/NetRatings. The top search providers will continue to compete for customers’ loyalty with new features, improved functionality and rewards programs.

In the past year alone, all three have succeeded in boosting their number of monthly searches. Google’s search count rose 34%from 1.9 billion searches in April 2005 to 2.6 billion in April 2006. Yahoo rose 27% from 919 million to 1.1 billion, and MSN rose 10% from 515 million to 570 million.

Those numbers can translate into revenue as increased traffic allows each search company to charge a higher rate for more exposure of clients’ advertisements.

That could be lucrative territory, since the top shopping terms entered on Google in April included retail giants like Home Depot, Wal-Mart, Target, Sears and Best Buy. Of course, pure entertainment still rules the internet; the top overall Google search terms for the month included Nick Lachey, Denise Richards, Miss USA, Cinco de Mayo and Bettie Page.

This landscape could begin to change by the second half of 2006, says Allen Weiner, an analyst with Gartner.

Search providers are moving from a “straight search” method based on algorithms and page rankings to a “social search” method that incorporates human knowledge and other people’s preferences.

Yahoo has made the most progress, with its Yahoo Answers site already running, while Google has lagged with its Google Co-op site still ramping up. Yahoo also has an advantage in worldwide traffic, with a greater web presence outside the US than Google does, Weiner says.

In the meantime, Google will rely on sheer numbers to preserve its advantage. “These three guys are like the three TV networks back in the old days, and right now, Google has the top ranking show. Until proven otherwise, they’re the leader,” he says.