It's actually pretty simple, you get additional points for different content and I try to scale it based on the difficulty of creating said content.
Going roughly from memory, this is the scale (or close to it):
1 point - get a reaction from someone
2 points - create a post
5 points - create a topic
10 points - create a blog entry
Then there's a bunch of random other stuff, too. If you hit a milestone like 1,000 posts in the MiLB forum, you might get 50 points or something like that.