How does Quora.com and Wikipedia measures their monthly active users?

I am keen to understand what are the key matrix of user engagement Quora.com and Wikipedia has? and How Quora.com and Wikipedia measures their monthly active users? as I understand every product has their own way of measuring monthly active user for e.g. Facebook has different way and Twitter calculate it in other way, but I would like to know how Quora and Wikipedia do this as major portion of the traffic to them is organic

Would really appreciate if anybody can help me understand this. Thanks