We're still trying to figure out exactly what Apple's new Ping social network is good for, but apparently quite a few people are curious about it: it's already racked up a million users in its first two days. That's just a third of the people who've downloaded iTunes 10, so we're sure the numbers are will grow even faster as more people snag iTunes and the network effect starts to kick in -- and things will get really crazy if Apple and Facebook ever work out their little tiff. Just remember: it's a social network... for music.