While there are tons of numbers to suggest that Facebook is sticky, that it attracts lots of people, there are lots of numbers we do NOT have. People are focusing on the easy evidence, which is only of tangential relevance for business.
We all know from the last ten years that high "engagement" on videos sites, forums, etc, doesn't generally pay off for the hosts. YouTube a good example, althought 2010 may be first profitable year.
As a business platform, facebook leaves a lot to be desired right now in terms of usability, so the race will be between how fast users and business get tired of it, versus how they can introduce better, easier to use features.