Thirty-four U.S. billionaires pledged Wednesday to give away at least 50 percent of their wealth to charity as part of a campaign by investor Warren Buffett and Microsoft founder Bill Gates. Among the billionaires joining the campaign are New York Mayor Michael Bloomberg, entertainment executive Barry Diller, Oracle co-founder Larry Ellison, energy tycoon T. Boone Pickens, media mogul Ted Turner, David Rockefeller and investor Ronald Perelman. Gates and Buffett launched "The Giving Pledge" in June to convince hundreds of U.S. billionaires to give away most of their fortune during their lifetime or after their death