A likely cause for this that I haven't seen noted yet is that they are slyly directing people to G+ without the user even knowing. If you click on a Google Maps result to get the full information page, it's no longer a Google Maps page, it's a G+ page. These traffic numbers clearly include lots of people who got to G+ in this manner.
Yep, first noticed this a week ago. Lots of services redirecting into G+. Definitely going to skew the numbers WRT the concept of active vs passive growth.
