Danah Boyd, of the School of Information Management & Systems at the University of California, Berkeley , is (as far as I am aware) the first person to define the term "friendster whore". She is researching Friendster and other social networks tools, trying to understand how people present their digital identity, negotiate social contexts and articulate their relationships.
Her definition, which I have adopted, is taken from her blog Connected Selves, September 1, 2003:
"Friendster whores - people who simply collect as many people as possible" (for further information about Danah's work please see the post for October 11th below, or visit her blog apophenia)
Update: Rich from the #friendster IRC network states that he came up with the term back in February (see his post below, Oct. 21st)