Buy facebook ad's
it works.
You can take it a bit further and really target your facebook ads by using the custom audience ad feature. You would need to get some software or scrape facebook group userid's that are relevant to your niche upload the csv to facebook and it will only target those people in your custom audience. i personally do this and have the software for it so you can hire me to do this
Example : Your niche is e-cigs/vaping. I would use my script to scrape 100k+ facebook users who are active in groups regarding e-cigs. I would upload that as a custom audience. I would set up a new campaign and make it use that audience. you are spending a few cents per like at this point.
Its easy if you got a small budget of a few hundred bucks