Brands typically complain about having to pay Facebook just to improve its "Reach," or the number of unique people that receive impressions of a Page post. But according to data from Socialbakers (chart via Statista), it looks like spending money alone doesn't equal engagement.
In fact, it looks like a Page's total reach on Facebook is affected by the quality of the original post, which is signified by its organic reach and number of interactions. When there are fewer interactions and not much organic reach, paying Facebook only helps just a bit - roughly 2.3 times higher than the organic reach, on average. But if a post has high organic reach and several thousand interactions, paying Facebook for a promoted post is extremely effective at reaching users.
In short, paying Facebook to promote one's posts largely depends on the quality of the post and the ability to generate interactions with one's audience.
Business Insider/Statista