There is a severe competition in Internet and daily you can find some 5-10 new sites in every categories, may be shopping, dating, ptc, social, information etc (almost every field) but the internet users are not readily increasing. As the new sites coming up, they come up with good offers to popularize their site and this drags people away from old to new site and the importance of old site decreases, so it is very probable that their PR will decrease or you may the importance shared with many sites and average PR of all sites lowered.