Moovweb has an article wondering if social sharing buttons on mobile sites are a waste of space?. The short answer is "yes", but I have a different conclusion from the one in the article:
The heart of the sharing problem is that users must be logged in in order to share. If you’re not logged in, sharing can be kind of a nightmare.
While the statement is true, for me personally I share direct from from the mobile OS. iOS and Android have social sharing built in (and have done for years) so why both even looking for a button within page when you have a native version in the same place it always is?