By ARIANA EUNJUNG CHA Washington Post Staff Writer    As the country's racial and ethnic makeup rapidly shifts from majority white to majority everything else, entrepreneurs have been rac-ing to build out online portals targeted at African Americans, Latinos and Asian Americans. The sites often feature news about minorities that may have been underplayed by the mainstream media. Many include historical information about immigration patterns and feature chat rooms where users can make friends with people of similar backgrounds.    While the stock market's downturn in recent months has caused some of the companies that run these Web sites to trim their staffs and expenses-one, Quepasa.com, announced yesterday that it would liquidate its assets-their user bases continue to rise. Of the sites listed below BET.com had 1.1 million unique visitors in November, BlackPlanet.com had 534,000 and StarMedia (www.starmedia.com) had 805,000, while the other sites below had 200,000, according to the research firm Media Metrix.