The more people do something, the more "normal" it becomes. Easiest way to spot a normalfag is if they do something everyone else is doing. Not even being hipster about this, it's just the way it is with the NORPs.
That's not the definition. The person who is least normalfag in existence breathes oxygen, which everyone else also does. To create a good definition you need to be stricter in your definition of "something," and you definitely need to add in a clause that not only do they do it, they attempt to impose it on others.