I've never used anything other than my good old spotify user (no facebook login). Yes, it's connected to facebook, but I don't use the facebook credentials when I login.
I'm also using spotyxbmc2, but that shouldn't have anything to do with it? Using the same username there, and it worked perfectly fine until around the date mentioned.
I often login on several clients at once. For instance playing from spotfyxbmc2 while adding tracks to playlist on the spotify client on my laptop. But since it's the same user account, this should't matter ... I think.
When logging in with the jkjk account (and I have no other as far as I know), I only get the few favorites I have added after the list disappeared, and those only. All my playlists and other setup are exactly as they've always have been, so no problem there. It's just the favorites that are gone.
I'm having the same problem, about half of my favourites list has dissappeared. Everything I starred before a date in 14/11/2010. I haven't used the playlist for a couple of weeks so it might have happened back then.
Hope you can get to the bottom of this.
Edit: The Problem has been fixed. If this was you then well done. Now I just have to resync 200 odd tracks.