You see that Google+ icon either on the left hand side or the top right of this content? Well, every time you plus one vote up this content, Google tracks it. It is supposedly reported within Google Analytics, no longer Webmaster Tools.
That being said, those who track their Google+ votes have been saying the number has been dropping significantly recently.
One webmaster said in a WebmasterWorld thread:
I was just reviewing my +1's on my one of my Plus Sites. Today It went from over 6000 down to around 360 +1's. Is this happening to anyone else? First it was WMTs links to the homepage not showing up, then the removal of a screen in Index Status, now this. They are on a mad tear lately.
That seems incredibly drastic!
There are plenty of "me too" posts following that post.
There are dozens of posts in the Google+ Help forum as well with the issue. There a top contributor said:
I think it's a glitch. Similar things have happened before, and the numbers usually returned to what they should be after a while. I have pages that are affected too.
No word from Google on if this is a feature or a bug.