One good way to determine which buttons to use is through your analytics console. By seeing where your visitors are coming from, you’ll know which buttons to use. For example, do you get a lot of visitors from Facebook? If so, add the Facebook button. If you get zero traffic from Digg, you might want to skip that button. Of course, if you are getting zero traffic from a particular site, you might want to add their button to see if you can generate some traffic from that site. If the button does not bring in new traffic after a month or two, you can always remove it.
Benefits of adding Social Sharing Buttons: