Their model seems to assume that Facebook accounts are something someone make one of, and when they're done with it, stop using it.
For a lot of people I think Facebook accounts really are transient ephemeral things more like colds.
Whenever when some damn website or game makes me have a Facebook account to sign up -- I make a new account with a throwaway username / password / email that I never care to remember -- and never use it again. That's why I think a lot of those "facebook has X users" or "Y% of users have abandoned facebook" are totally bogus. For just my accounts, sure I've abandoned 90% of them. But that doesn't make it fair to extrapolate that 90% of facebook accounts get abandoned. Just that some people don't want a permanent Facebook account.
TL/DR: I do get facebook accounts very much like I get mild colds. A get a new one a couple times a year; it doesn't last for more than a couple days; and they're merely mildly annoying.