Five possible reasons for forum quietness:
1) Work-life changes. (Many who used to post on here have grown up, got families of their own, and spend their free time on other things. Others may be spending more time working hard to try to improve their situation.)
2) Social networking sites changing how people discuss things on the internet (killing forums in other words)
3) In the case of the World forum, a feeling that debates there have been done to death over the last 10 years (and a growing belief that interwebz debatez are a waste of time, perhaps?)
4) A lack of new members.
5) People got fed up of reading all those stupid/overlong/nonsensical/tedious posts written by ??????? (insert forum member name of choice
)