Adding back Pinterest sharing count

This re-fixes Automattic/jetpack#597. Was fixed in Automattic/jetpack#749 and appears to have been undone by a wpcom merge via SHA: 7c7b1de409

Checking the wpcom merge (r101459), it looks like this wasn't intentional. Since it is a regression, I suggest this be considered if another 3.1.x point release is needed.