Mary Hodder has a nice overview of how various blog tracking services work. Be sure to check out the feature comparison chart, but the key point to take away is that services like Bloglines, Blogpulse, Technorati, PubSub, Feedster et. al. are barely in the same business as Google, Yahoo!, and MSN. Sure various of them take stabs at keyword searching of posts, but their bread and butter is in trend tracking; mainly via link watching but with the potential for more sophisticated analyses down the road.