Ads have a conversion rate, and PR has a conversion rate. When you purchase a facebook ad that 100 million people see, you will expect to get a certain amount of sales as a result. The percentage of people who see the ad and proceed to purchase is known as a conversion rate.
When a product gets written about in several publications – and 100 million people see it – would you expect to get a certain amount of sales as a result? The answer is yes, and at a much higher conversion rate. Think about it, if a new tech gadget gets featured by CNN, The New York Times, Buzzed, and Business Insider — then 100 million people would see that gadget. Those people would be much more likely to purchase that product than if the product was seen by 100 million ad impressions. Thanks to the prevalence of ad-blocking software, and declining engagement on facebook (especially with millennials) – the conversion rate from PR is higher than ever compared to advertisement.
Seriously, anyone under 30 no longer uses facebook – and anyone with a bit of tech savvy uses an ad blocking chrome extension. Thus, PR is literally the only way to reach many market segments. This is especially true for reaching 20-30 year old males.

Fast Company’s notes on PR vs. AD conversion rates: