The most effective placement of social sharing buttons is the place, at which visitors make the decision to share and where it's the most prominent.
Naturally users will decide to share once they're done consuming the content. This means that the social sharing buttons should be located somewhere close to the bottom. However, content footers often contain a whole lot of meta information as well as links to "related" content and whatnot. All of that is competing for attention with the social sharing buttons. In addition, visitors won't know what sharing links are available until they scroll all the way to the end. Thus, it seems that they should be present at the top as well.
A less obvious advantage of top placement is that high sharing numbers serve as a form of social proof encouraging people to read & share the content when they're making up their minds.
AddThis (one of the leading social sharing toolkits, which I shamelessly promote because they're also from DC are) has these suggestions in their best practices for placement:
- Keep AddThis near the top of the page
- Place sharing near the content being shared
- Don't put sharing in the navigation
- Avoid putting sharing in the footer
- Avoid putting sharing below the fold
Their best practices also have an image with a sample layout and the instructions that read, Install AddThis before and/or after each post (see the link above).
Another set of placement recommendations from them suggests (italics added):
- Keep your button near the top of the page: Avoid making your readers scroll to find your sharing button. It is okay to have the button at the top and bottom of the page, but users will find it easier at the top.
- Watch out for navigation: be careful about placing the button too close to navigation, so users don't interact with it by accident.
Therefore, don't try being cute or gimmicky and put sharing tools both at the beginning and at the end of your content (just make sure they don't distort balance when everything is visible above the fold).