IMDb keeps wiping her board clean

Yeah, they do that with some boards. I don't get it.

IMDB regularly deletes the older posts from more popular boards:
If you are certain that the post has been deleted and you didn't do it, then there are two possible reasons: the message has been removed by a moderator or it has expired. Our boards are not designed to store messages indefinitely: after a certain amount of time older threads are removed from the board automatically. This process is called "expiry".

The rate of expiry is adjusted dynamically according to the posting traffic for each board. Threads are removed sooner from a board that attracts a lot of posts than they would be from a board with less activity.

Please keep in mind that the boards system is not meant to be an archival medium, and all posts are subject to expire and disappear sooner or later. This is intended to allow us to manage the resources consumed by each board effectively, and also to promote the ever changing flow of discussion.

http://www.imdb.com/help/show_leaf?boardswhydeleted