The only true way to measure its effect, is to take it away and see what happens. Of course, other sites would likely pick up some of the slack, but that all depends on what you mean by presence.
The title says presence, yet the post says importance. Those are two very different things. Things can be important with no presence, and things can have a lot of presence, without being important.
Perlmonks may not have presence due to other sites that google promotes, yet i don't think that can measure it's importance. That would only come from taking it away and seeing what happens.