A 500th post is special on GameFAQs because a topic is locked at that point. Getting a 500th post is seen as an honour that can be bestowed. Normally in Social topics it is left up to random who gets the 500.
In game topics and particularly the Emotional Help topics, it is normally specified that the 500th post goes to a specific person. It is severely frowned upon to ignore any stipulation. It is also frowned upon for a user to "snipe" the 500th post after not participating in it as Vujman did on Christmans Eve 2005, three times.