The only topics that we consider are the ones in the "Social" section of the forum, and never the informational topics in the rest of the forum.  Even then we are usually only looking at the topics in "Random Ramblings" and "Jokes and Games" area.
One of my top priorities from the beginning was to ensure that we can store all the fantastic and helpful information indefinitely while providing a very fast and efficient user experience.  Of course to help make this possible means that from time to time we may need to cleanup some of the longer random chat threads (again, none of which are in areas outside of the "social" section of the site).