Earlier today @ti-roberts posted some great website sharing tips found here http://goo.gl/KMI0D

We operate an eCommerce site which is a different animal than a content site so we look at plug-ins differently.
The article brought to mind a plug-in we use on our site here’s a link to their site http://www.addshoppers.com/ (no relation to the company) which incentivizes (is that a word?) sharing by offering coupons.
It has REALLY robust reporting and is free.