Every so often Matthew needs to clean some stuff out of the database in order to keep CI running without interuptions. Likers and critics take up room in the database, as do pm's, posts, etc. He clears them out to make room for other stuff. As I understand it, your posts will not disappear and your reputation points won't be altered. If you recieve quite a few personal messages, you'll probably notice that some of those have disappeared as well.
I was wondering when Matthew would do a "cleanup" the other day and figured it would be coming soon, so I saved a handful of personal messages that I didn't want to lose to my computer! I'm glad I did!