King of the Hill is one of my favorite shows of all time and I was actually considering making a thread about, so I'd like to give kudos to
@Jasonfan89 for this one.
I don't think it fell off in the same way that Zombie Simpsons or post-revival Family Guy did, but I do think there was a noticeable decline in quality from around Season 9 onward. There were a lot of layoffs in the old writing staff, which is why the episodes from that point on became a lot more episodic and less consistent with continuity.
Even then, it was only a slight decline and the weakest seasons were Season 12 and 13 which got produced on a last-second notice and a shoestring budget. Season 11 was originally meant as the final season with Luanne's wedding as the grand finale.
I like propane and propane accessories.
Butane is a bastard gas
Why was the show canceled anyway?
Really, it was a variety of factors. Around the mid-2000's, Fox was shifting their channel in a new direction and thanks to the infamous NFL contract they had back then, the seasons wouldn't get started until around the start of November to the point that The Simpsons had to start nearly every season with their annual Treehouse of Horror special and usually wouldn't get to air it in October.
King of the Hill got hit really hard by being preempted for football games under this contract so the ratings took a hit. This was one of the things that helped kill Futurama in its initial run during the early 2000's/
Most of Season 10 was comprised of episodes from Season 9 that were leftover from the year before, with only a few newer episodes produced to fill the gaps. So they decided that Season 11 would be the last one.
Then at the last possible second, Fox unexpectedly renewed them for another two seasons but on a lower budget. This is where you really see the majority of the show's weaker episodes, but it never got as bad as Zombie Simpsons did. The MySpace episode was from a script written back in 2005 but wasn't produced and aired until 2008, right when MySpace was in its death throes and Facebook was in ascension.