I noticed that it seems to go up relatively quickly at first, perhaps based more on frequent logons.. but my guess is that at around 100, it might start to be based slightly less on logging on, and slightly more on having questions rated, or sending questions to friends, etc.
again.. that's only what it appears to do.. just because people seem to slow down for a bit around 100.. but who knows. the pattern could just be an epiphenomenon, for all I know.