The parameters as far as I know are : # ACD Rank
User ranking on average comments received per day
# AIP Rank
User ranking on average interaction per post (includes comments and likes) This is the single largest contributor of Chatter Rank
# CC Rank
User ranking on comments they create
# CR Rank
User ranking based on comments received on post
# F Rank
User ranking based on number of followers
# Likes Received Rank
User ranking based on number of likes
# PC Rank
User ranking based on posts on personal wall
# PR Ranking
User ranking on posts on personal wall by others
# Ranking
Overall chatter “influence” ranking
# Chatter Rank
Score from (1-100) like grade school
You can read more on the discussion of this idea:
http://success.salesforce.com/ideaView?id=08730000000gQWaAAM. Those are the parameters as far as is known, however the weights and the specifics are not public knowledge.
--KC
