I've been using last.fm for a long time and I've spent a lot of time looking at the data. I would argue that the average person on last.fm is much, much more likely to have heard of Opeth than the average YouTube or Spotify user. I'd also argue that straight up listener count isn't a very useful number, since Kanye West (the most listened to artist on last.fm last week by about 33%) only has about 4M listeners. I don't think Kanye is only 5x as popular/successful/listened to as Opeth.