Any way to find out their methodology?
I'm a bit suspicious of diablo 2 being on the list. I played on the realms not TOO long ago, and it's relatively quiet these days. Might be interesting to compare Nielsen's Runescape rankings with the active player counter on runescape's webpage.
Either way, nielsen ratings aren't all that important with PC games. Companies themselves have a much better idea of what kind of revenue and userbase they have compared to tv networks.