nticaric

Quoty is a simple plugin that lets you share any text selected on the page. 
Currently supported platforms are: Twitter, Facebook, LinkedIn, Pinterest and GooglePlus
Features
<ul>
<li>Automatically displays a nice pop-up menu on text selection (highlight)</li>
</ul>

Quoty

Great!

Bravo momci!

A simple plugin for sharing selected text to social networks.

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

Quoty vs Sharect WP