If the whole music thing doesn't work out, Bono can always fall back on his mad skills as a businessman. Back in 2009, the U2 frontman invested $90 million into the site, resulting in a 2.3 percent stake. When Facebook went public on Friday, that $90 million turned into $1.5 billion. Not a bad return. Even two of the world's most successful entertainers combined, Bruce Springsteen and Madonna, don't come anywhere close ...