It is actually 500 posts to get the "#GhostArmy" title and becomes "Dedicated Minetimer" once you reach 1,000 posts.
After you go beyond 1000 messages, you earn the title "Dedicated Minetimer." If you would like to make your own, purchase Premium rank on the forums.
To add on to what TheWrongProblem said, if you do spam posts, those posts will be deleted and you have the chance to be banned from the forums.