Let's assume last.fm did indeed provide information to the RIAA. Won't we find out the truth once the RIAA starts suing people based on that information? Surely last.fm would plan for something like that instead of issuing denials.

This leads to one of two conclusions: either (1) last.fm isn't guilty, or (2) the RIAA doesn't intend to act upon the information they got. If (2), why did they request the information in the first place?

Personally, I'm inclined to believe the TechCrunch story is nothing more than linkbait, and last.fm is trying to give them as little attention as possible.