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: http://bit.ly/rlq9aR
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