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.