I thought I add a few discussions that I have come across in my research:
Some stats and opinions on sharing buttons:
http://ux.stackexchange.com/questions/4727/are-share-this-buttons-really-effective
Suggestions where to put the button depending on the content of your website:
http://ux.stackexchange.com/questions/18047/share-buttons-on-landing-page-where-to-place-them
Understanding the difference between social buttons:
http://www.streetarticles.com/social-media/where-should-you-position-social-media-buttons
Discussion on the most effective location of social buttons:
http://www.ixda.org/node/29819
http://ux.stackexchange.com/questions/3462/social-share-buttons-best-position