nektobit

A lightweight (8Kb minified, 2.9Kb gzipped) script to let users share their text selections to social networks, in desktop browsers. (Yes, like Medium). 
Based on Sharect.js
<ul>
<li>Supports adding custom icons (for now using php snippet)</li>
<li>You can also control the color of the tooltip widget (also using php code)</li>
</ul>
<a href="https://nektobit.github.io/WP-Sharect/#documentation" rel="nofollow ugc">Documentation</a>
Millstones
The plugin is under active development, I welcome any feedback.
<ul>
<li>Add widget color picker</li>
<li>Adding social media icons from the admin interface</li>
<li>Optional meta tags</li>
</ul>

Sharect WP

The plugin allows you to highlight text on your website and share it on social networks

Sibin Grasic

Select to Share WordPress plugin will allow you to easily display a nice pop-up menu on text selection. 
CSS selector option is provided so you can target only your post / page content.
Features
<ul>
<li>Automatically displays a nice pop-up menu on text selection (highlight)</li>
<li>Mobile fallback with pop-under sharing menu</li>
<li>Highly compatible – you can change the css selector for the text highlight</li>
</ul>

Select to Share

Nice

Show a nice pop-up menu on text selection for sharing on Facebook, Twitter and e-mail...

Sharect WP vs Select to Share