I know Board Tracker has some sorta of formula in place now for their rankings that, as far as i can tell, is based on posters, threads, replies & views, each being issued a number & the total used in the ranking - of course that is just a guess
The only true way to do it is via a 3rd party counter, but even then, you'd have people slapping the counter up on pages that aren't part of the board just to see the nubmer rise up a bit.