Here my ideas:
1 Logins per month (e.g. 10 logins = 100% activity, 9logins = 90%...)
->> I would say if you login every day = 100% everyday-1 = 97 etc. (No problem to code)
2 Page clicks per month (50 clicks= 100%, 25 clicks=50% ...)
->> I would say the person who clicks the most = 100% (No problem to code)
3 Profile completion bonus (plugin should count the available profile fields and then the fields that are not left empty and generate a percentage value that gives a bonus for activity level)
->> We also have to take into account the field that are hidden oder not editable... (should be doable)
4 forum posts: count the forum posts in FB and give a activity percentage bonus for each post in the last 3 months or so.
->> most post = 100% (No Problem to code)
5 count number of new connections over the last 3 months and give activity bonus for each connection
->> this will be the tricky one, because I do not use connections.
6 count group memberships in Groupjive and give activity bonus for each membership in a group/ count own groups as founder and give extra bonus for each group founded in the last 6 months or so
->> this will be the tricky one, because I do not use Groupjive.
7 Count CB Gallery items and give activity bonus for each new item in the last 3 months...
->> should be tricky but doable.
The user should be able to choose which he would like to use.
I think I will start working on the Logins per month and page clicks first, after I got your imput.