Plug-In for automatically distributing new posts to social media sites?

I’ve been reviewing the various social media plug-ins and, as far as I can tell, most (or all) simply offer the visitor a convenient icon to bookmark a post, if they so desire. I’m looking for a more “active” plug-in that will automatically send each new post to various social media sites. Is there an available plug-in that does this, or is there some third party software that I should look at?