I wonder how many of the 61 million visits have been pushed to Google+ via SERPs?
IMO Facebooks lead in market share here is much higher than those figures suggest. There's a vast difference between someone going to Facebook through choice and ending up on Google+ because they've search for something and it's been forced in front of them.
These days I've been taking any Google PR efforts with the same pinch of salt that I give to Adwords keyword tool search volumes. ;)