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.
