In order for a Facebook ad to succeed, it must target the right people. If you trust professionals in the field, then this type of advertising will invariably achieve its objectives. If your ad is shown to people who are not interested in the product or service, you will spend a large budget without making a lot of real sales.
You should keep in mind that possibly mis-targeting your audience can generate a lot of likes, posts, and comments, but in most cases it doesn't help at all in boosting actual sales.
However, if you have a high volume of likes, posting would not be completely useless, since sharing it with friends or other public and closed groups increases the social activity of the ad and thus the cost per click decreases.
Referral from a previously shared post to a user is free of charge and here comes the moment when the ad is distributed free of charge as a result of sharing by a user.