I have to say one thing on your response, forget about tipping anything on this score, Social sharing is the new Google, thats why Google brought out plus to try and take on Facebook & Twitter because an increasing amount of site visits now seem to come through the social networks, so placing Facebook, Twitter and Google+ sharing plugins is a must for any serious website, that is where the people are spending there internet time so it makes sense thats where you want your website to show up the most.
Of course there are those that will continue to use search engines, but look at it this way, If their friends are posting about your excellent products, When they see your website on the search who are they going to click? An unknown entity or You who their friends have already endorsed by sharing?
The main thing i guess here is only having the sharing links on pages you want to be shared. this way you can at least have some control over what if anything at all gets shared.
(sorry if i seem a little off.. long day today and its only 11.45)