I'm sorry, but is this really just a handful of anecdotes about individual people who have stopped using Facebook? And this proves what, exactly? Every successful site has had people along the way that stopped using it for one reason or another.
Come back when you have some data on either the number of people leaving or the impact of their absence.
I think the data won't be so easy to collect. I think about 2/3 of my 'friends' have just stopped using it. Their profiles are still up, maybe they sign on once in a while to see if an old friend is trying to contact them. Frankly the article corresponds exactly with my experience.
A poll by an external agency wouldn't be very representative of Facebook's population, so Facebook is the only organization that has access to data to accurately make these kinds of claims, which they're going to hold very close to their chest.
Therefore, the obvious easy way to collect this data is to wine and dine some Facebook statisticians :D.
Come back when you have some data on either the number of people leaving or the impact of their absence.