I will give you 4 ways to monetize and sell your plugin.

First of all, I need to say that you are using social media and Adwords which is fine, but getting the traffic to your plugin page is the challenge. And if you are using social media, are those people in your target market to even want the plugin? They must be WordPress site owners and developers, or otherwise all the Adwords and Facebook advertising will not work.

First Way:

What has worked for some of my clients is to offer a LIMITED version of the plugin for free and then offer an upgraded version for your price on the WordPress repository. That is what the most popular plugins do.

WooCommerce is free, but they charge extra for the add-ons, so that is one way you can go: Offer your plugin for free and limit its functionality and then offer paid upgrades for extra services.

Second Way:

Another way to monetize your plugin is to do what most of the free plugins do: Offer about 50% of its functionality for free and then the other 50% is available through a paid upgrade which is your regular price.

Third Way:
Offer the full plugin for free and ask for donations.

Fourth Way:

This is what I do. Attend as many WordCamps as you can around the country and hand out a flyer or business card with your paid plugin address on it.

I have seen all 4 of these suggestions work for various plugins. I hope one of these works well for you.

If you have any other questions about this, please let me know.

Bruce