General Discussion

Most of the people Limbaugh attacked were public figures, and in the US it is very, very hard to get a defamation judgement against a public figure. I believe the reason he eventually half-apologized to Sandra Fluke is that he realized there was a non-trivial chance that, even though she had testified before Congress, she might still be considered a private figure, because if simply testifying could strip you of your privacy protections nobody would want to do it.