Thats pretty much it - seems to be time logged in + post count up to a max of +14 over a two week period. So you could write 14 messages in one day and have 14 as the activity but then not gain anything further for another two weeks, even if you post loads of messages. I think! I haven't done that much, I don't want to spam and definitely do not want to get on the wrong side of the Mods!