Pay With A Like - WP Plugin

Simply allow visitors to exchange a Like, +1 or Tweet for post content, videos, a free download, a coupon code or pretty much anything you want.

Then, by clicking Like, they’ll promote your site to their friends, followers and colleagues… who may also want to pay with a like.

And the more likes, +1s and tweets you get – the higher your post and sites will appear in Google, making this amazing for SEO too!
Social Share Wall

Essentially you are protecting your content behind a Social Share Wall, like a paywall (where your readers pay to access content) except you get them to share your website with their friends and colleagues instead of asking for cash.

Its the smartest way to get your site out there, drive traffic and push yourself up Google (which takes these kind of social signals very, very seriously).