Couple things:

1) Ping is only available in the US, so a straight comparison of user numbers to FB isn't necessarily valid. You have take the number of US users of both iTunes and FB, and I don't think that breakdown is readily available.

2) Non-mainstream stuff is likely to be overrepresented in these sorts of things, since people who are into to music are more likely to use such new music services and are also likely to have more varied tastes.