This has been our situation = MSN had been hitting the site aggressively prior to their search engine launch. When the launch happened the site had about 8,000 URLs in the index. While this seems like a lot - they actually spidered a lot more URLs than that (our site has 50,000 URLs). From then on the number of indexed pages changes quite frequently both UP and DOWN constantly. Looks like MSN does not have any 'memory' of the URLs it spiders and must re-spider pages constantly to have a representation of a site in its index.