A Facebook account with 1m fans generates an average of 826 likes and 309 comments per post

"Social media brand monitoring platform, Simplify360, explored the relationship between the number of Facebook fans and engagement level to reveal that on an average, each new post generates 826 likes and 309 comments.
The research firm took 50 Facebook fan pages with a random mix of brands from all over the world from consumer brands, to sports teams, to celebrities."

Source:  Data from Simplify360, reported by The Next Web, 17th May 2011