Google extended its lead in U.S. search engine usage in April for the ninth consecutive month, while its four closest competitors all lost market share, comScore Networks said Monday.Google handled 43.1 percent of searches conducted by U.S. residents in April, up from 36.5 percent in April 2005.Yahoo followed with 28 percent, down from 30.7 percent. This translates to 2.9 billion searches for Google and 1.9 billion for Yahoo, a gap that has widened consistently for the past nine months.Then came Microsoft with 12.9 percent, down from 16.1 percent, Time Warner with 6.9 percent, down from 9 percent, and IAC\/InterActiveCorp\u2019s Ask.com with 5.8 percent, down from 6.1 percent.Attracting users translates to advertising revenue for search engines such as Google and Yahoo, which serve up online ads along with search results. The search segment is the largest of the fast-growing online ad market, accounting for 41 percent of revenue. Online advertising spending grew 30 percent in 2005 to US$12.5 billion.This is why Yahoo, Microsoft and others have invested heavily on their own search engines and search-based ad networks. However, despite their efforts to strengthen their positions, Google is fending off their challenge.While it increases its dominance in the search engine space, Google is attempting to make inroads into the Web portal turf of Yahoo and Microsoft\u2019s MSN unit by offering services such as Web mail, blogging, photo sharing, RSS reader, online calendar and instant messaging.Most of Google\u2019s ad revenue comes from search engine-based ads, but the company has made no secrets about its plan to diversify its business into sales of display ads, such as banners, and multimedia ads, like those containing video and audio. These ads are more commonly found on Web portals.-Juan Carlos Perez, IDG News ServiceFor related CIO magazine content, check out The Enterprise Gets Googled.For related news coverage, read San Francisco to Push Google on Privacy and China Mobile, Google Talk Cell Search.Check out our CIO News Alerts and Tech Informer pages for more updated news coverage.