This plugin lets you add social media sharing buttons to your pages and posts. You can put them inline at the top of your pages and posts, at the bottom in a call to action box, and/or as a floating share bar that moves as you scroll. Many social media accounts are supported. There are a lot of customization options, such as button type, button order, and showing or hiding certain buttons on a per page/post basis.
More information: https://wordpress.org/plugins/socialize/