Correct answer: Let your customer share however they want, wherever the want. It’s all good and it all brings you new visitors.
Your job is to make sharing as easy as possible and get out of the way.
(If you want to know what’s hot, look at the default settings on services such as ShareThis or AddThis. See the photo to the right: ShareThis is offering Fousquare and Pinterest, and we don’t see much of Digg or Reddit.)