In the mid-1980s, “Do They Know It’s Christmas?” and “We Are the World” gathered dozens of the biggest stars in music to put on a show for a good cause. The two songs spawned imitators, but today, the charity megasingle is a relic of pop music’s past, except around the holidays. This month, we examine how good intentions, pique, excess, and vanity led to the rise and fall of the do-gooder celebrity pop song.  Email: hitparade@slate.com  Learn more about your ad choices. Visit megaphone.fm/adchoices