Not to mention if you care about the privacy of your users every share button is a little tracking device that probes into your users habits and pages visited. (Ghostery can help you see the extent of the tracking that is taking place)

I get really mad at share buttons because there's no way to know beforehand if you are going to get to a page filled with those.

From observation I came to a simple law about these share buttons: the amount of content is inversely proportional to the number of share buttons on the page.