I propose the gravitational theory of FriendFeed. If a post has a small number of comments from some "big names", and the object is wandering in the right part of the universe at the right time of day, it will gather up some other interesting comments, which makes the post bigger, which gathers up a huge number of other comments.
When do the comments stop piling on? It is when the object wanders off into near empty space-time. The comments will also stop gathering on that object when another larger object comes into view, and people start commenting on more interesting "matters".