Hello,

It all depends on what kind of sharing do you need! If you are looking for your users just to be able to share the content from your site there is no need for a plugin really. You can do that with just embedding few lines of JavaScript code in your templates.

On the other hand if you need to share user activity for example when user submits link/review or save a link in his bookmarks I'm afraid not all the sites you've mentioned will allow you to do that.

It is possible only with Facebook and Twitter, the other two (Pinterest and Google+) do not provide an "write" API with such functionality ... in fact Pinterest doesn't have an API at all, so you are out of luck there, G+ on the other hand may or may not open their API for write access, as of the moment their position on that matter is "we are thinking".

I've built sharing functionality (with twitter and facebook) for one of my sites but never had the time to wrap it in to a proper plugin.
If Facebook and Twitter are enough for you and you are interested drop me a message.

Cheers,
Boris

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