I guess we don't share that officially because it always comes with breaking other user's code when we change something. Defining a stable API is hard work and can be very limiting.
I see what kurp is saying, like giving favorites and downvotes a value, and make the rank affected by how much your stuff is liked by the community, and use it to determine something like how high up the list of artists new users see when they join.
On a somewhat related note, what format are the timestamps for registered/last online that are also found on that page?