On one front, you are absolutely right Adam - brands should have nothing like as much expectation as artists so earn any kind of engagement from fans.
What is quite telling however, is just how few brands ARE in the top 100 even. I am currently looking at how feasible it is from a time point of view, to look at the Top 100 social brands as determined by Altimer Group and run theiur figures through the tool. Each report takes quite a while to process (especially at the top end of the scale), so I just need to find a bit of time to take a proper look at it.
From an engagement point of view, I agree, average engagement scores will always look weak the larger the page, but I htink there is a blindness to Page owners when they see a post gets 5,000+ likes or comments - in the scale of the volume of the page, this number is significant.
I hope that the proper, sophisticated facebook management platfors will soon be able to categorise posting content according to the type of person who clicks - giving Page owners a much better chance of creating content that gets clicked on more than this single time.