I don’t know how the phrase developed. If I had to guess I would think a tap is when you listen in, like when your phone is tapped. So, if you think of the collective as a bunch of people having comments and conversation, and then if you listen in you hear all that is being said. It’s like something already exists and when you tap in you get to be a part of it. I am not explaining well.
I googled and got this recent article about the elections in NY, they us the expression in the title http://www.nydailynews.com/opinions/2009/10/15/2009-10-15_no_cakewalk_for_mayor_mike.html now you will probably notice more when it is used.