[giantrobot]

To expand on Dejanews' search (for the uninitiated), Usenet servers had limited retention. That was the length of time they'd keep a message on the server before deleting it. If I sent out a message a few months ago and you just checked the server today, it's entirely possible you'd never see my message. You might see more recent replies to my original message but your server might not have my old original message.

Because every Usenet server carried copies of messages each one determined its retention time based on the amount of storage the machine had.

Dejanews imported a bunch of old Usenet archives and ran their own servers which subscribed to everything and indexed it all. They were to Usenet what Google was to the web.