How do the rankings work?

This is described on the Top 10 Users page if you actually read it.

The scores are a calculation of how actively you conrtibute to the community. Musings and Comments are both worth a fraction of a point, News Stories are worth 3 points, and the amount of times you've logged in is worth even less than Musings and Comments.. Journal entries don't count for anything.

UPDATE: Threads are worth more then musings/comments. Polls are worth more than Threads, but less than News Stories.