AFAIS, they don't remove players from the game unless they wish so. Others stay in the list though they are deadwood. To the company, it seems paramount to have all of these count to the big number of participants - bragging may rise the ad revenue, and some players find pleasure in bragging about their friends' number...
Furthermore, what would such automatism add to their revenue? Nothing, it would only cost precious programmer's time needed for those gimmicks!