If you add a Google +1 button to your website, Google Analytics can automatically track how site visitors are using that button without you having to modify the code.
Alternatively, you can use the analytics services of external social sharing platforms – like AddThis or ShareThis – to record interactions that people are having with the Google+ button (and other social buttons) on your web site.
Google’s button policy document however discourage that approach:
Publishers may not sell or transmit to others any data about a user related to the user’s use of any Google+ button.
For the avoidance of doubt, this prohibition includes, but is not limited to, any use of pixels, cookies, or other methods to recognize users’ clicks on a Google+ button, the data of which is then disclosed, sold, or otherwise shared with other parties.
When you implement Google+ on your website through a third-party widget, it will most likely track the clicks on that button and Google’s policy clearly prohibits that activity.
I find this clause strange because when Google introduced social tracking last year, they kind of encouraged developers to build this kind of tracking on top of Google Analytics but the button policy says otherwise. Or may you can only use Analytics for recording the +1 interactions that occur on your site.