well the argument of number is tricky since nobody has the number except Google. But another product of google's own, Google Trends, do help to see the decline. See here:
I don't think pageviews per visit is anywhere near what it is in Facebook - probably even by an order of magnitude. That is just my own perception from using it, though.
Facebook really do know how to suck time out of their users.
And the even sadder thing is, even if "Google+ now opens to everyone", the search term of google+ has retreated to near the low level of post-launch. sigh ...
a more accurate graph is:
http://www.google.com/trends?q=plus.google.com&ctab=0...
and that only shows up until the end of Sep and doesn't include the few days last week that google+ was linked to from the homepage of google.com