I have come up with three different theories about why Fox would put Family Guy at 8:00 on Thursdays:
1) Fox has lost all confidence in the show and have no plans to renew it. However, they are commited to it for this season. Expecting the show to get low ratings, they've stuck it in the one slot where they always lose anyway.
2) Fox thinks people are getting sick of Freinds and Survivor and think they can steal some of their audience away with what is unquestionably the funniest show on tv.
3) Thanks to our letter-writing campaings, Fox is aware that Family Guy's fan base is extremely loyal. This means that they can air the show whenever they want and more or less expect the same ratings. Since they can't win on Thursdays anyway, Family Guy is a perfect fit. Even though the show's ratings are only ok, they are still much better than anything else they could air.
Actually, if #3 is true, then the Fox managers are quite intelligent. If #2 is true then they're a bit too optimistic, and if #1 is true then they don't know good tv when they see it. (This a bad trait for a television manager to possess. ;)