Interesting problem and one that I have not come across before now and I have DropBox installed on my three PC's. I have actually moved my Favourites folder into DropBox and have just saved a site and yep it has appeared on the other two PC's Favourite folders also.
Have you reached your 2GB free quota? is my first thought. If so it is pretty simple to increase this if you go to the DropBox website it will tell you how.