Someone who posts large quantities of posts that usually do not have much meaning. Could be to feel that they have some sort of authority, have no real life besides the message board, or get some special recognition for it. It also could be all three of those or many more reasons.
Mistress of Light on Deathrock.com is a postwhore. She believes in quantity not quality and makes it a point to post on every thread in existance on the board it seems.