The short answer is that any NPO which is serious wrt driving social media engagement, will identify and track a number of metrics. Usually this involves predominantly Facebook and Twitter and can be achieved by having a suite of metrics in place-
Not only NPO/NGO, any organization track carefully bulk of metrics available on social media. Based on those metrics or KPIs [if you wish], they could conclude about the overall social media effectiveness. Basically, in digital environment is easier then ever to trace your performance.
Deborah already provided you with invaluable URL whereby everything is very nice summarized and elaborated.