The following graphic looks at all World Series ratings since Fox made the change to the current World Series scheduling in 2007. That schedule has games on Wednesday, Thursday, Saturday, Sunday, (then if necessary) Monday, Wednesday, Thursday. Since then, no Series has gone the full seven games.
First observation: despite a somewhat wide spectra of ratings among these last four World Series, they've all gone down the same trajectory game-by-game. Decent Game 1 sampling, a dropoff for Game 2, a series low on the low-HUT Saturday Game 3, a spike for Game 4, a drop for Game 5, and a spike for Game 6. The 2010 version actually saw the smallest Game 5 drop, but the pitching matchup and the fact that people showed up as the moment of elimination neared probably helped that.
Second observation: among the last four World Series, there have been two "haves" and two "have-nots," and it's not a coincidence that those two teams I mentioned at the top were involved in the "haves." People aren't kidding when they talk about the ratings bumps those two teams bring.
The 2010 Series had, game-by-game, the worst (or tied for worst) rating of any of the last four until its Game 5 narrowly beat the 2008 Game 5, again probably because of the pitching matchup/threat of elimination. And the fact that the 2008 World Series was marred by rain delays only makes the 2010 one look even worse.