I quickly went back to my library, right clicked on a song and searched for some Post button. Unfortunately, as I was soon able to find out, you can only do that inside the ITMS. Of course, some of the songs I have on my library are not in the ITMS, but they could still figure out an automatic way of matching the artist and track name, ask me if one of the matched songs is the one I'm talking about and post it.
As a result, Ping, instead of being that brilliant last.fm killer idea we all envisioned when we heard about it of bringing a music social network to the actual music player people use, is just a webpage poorly stamped to my music player, with absolutely no integration with my library.
Even Last.fm, a 3rd party social network has more integration with my library then a social network inside my music player. If this makes sense...
P.S.: what I said about Posting is equally applicable for Liking a song. You can't do that in your library and there is also no relation between your 5-star rated songs and the music you actually like according to ping.