The methodology is basically using the Facebook Ad Tool. The tool let's you target a set of users for your ads and tells you the size of that population.
The article clearly discloses the inherent issues at play.
> Going forward, we’ll be watching closely to see what longer-term trends emerge. Bugs in the Facebook advertising tool that we draw this information from, seasonal changes like college graduations, and other short-term factors, can influence numbers month to month and obscure what’s really happening.
Other factors like Facebook finding and removing spam accounts, users enabling privacy features, etc. seem like a much more likely cause.
I could see people getting bored / busy and not bothering to use it any more, but it's unbelievable that 1.5 million would delete their accounts in one month.
The article clearly discloses the inherent issues at play.
> Going forward, we’ll be watching closely to see what longer-term trends emerge. Bugs in the Facebook advertising tool that we draw this information from, seasonal changes like college graduations, and other short-term factors, can influence numbers month to month and obscure what’s really happening.