According to UrbanDictionary:
POSTWHORE: Postwhore, or "post whore" is a term used widespread on internet forums to describe a person who measures seniority in a community by one's numerical postcount.
Often confused with "attention whore", a postwhore is marked by one's belief in postcount as a caliber of rank.
Used widespread on internet message boards, the conjunction of the two words: "post" & "whore" was established to define what is a postwhore, as we know it.
a) A postwhore is not necessarily an attention whore.
b) In some communities, the postwhores will only accept you into the community after you have reached 1000 in postcount.