Total number of accounts aren’t really indicative of anything more than unused profiles, at the minimum.
Facebook actually has to show how many eyeballs see ads and the dormant accounts wouldn’t go into that profile.
Which is easy to figure: the number of page views from unique IP addresses per day.
"Dormant" accounts creating bad traffic estimates aren't the real problem. The REAL problem is that FB can't make any money off of the petabytes of fake data that is stored on their servers. They have to pay to host it, but they can't sell it. Worse (for them), any data which do sell must be carefully screened and filtered, which costs additional time and money, and there is still no guarantee that the data is any good.
The solution to this problem is to NOT have selling your user's private information to third parties as the chief cornerstone of your business model.