It is common knowledge that a Facebook page does not reach its full audience, which can be considered as the number of people who like the page, formerly called "fans". Most of the time, a post will be seen by 10 to 15% of this number. This reach is called organic or natural because it is not paid. If going for promoted posts is the easiest way to reach a larger audience (which can go far beyond the number of page's likes), there is also a way to go naturally (or organically) above the 15%.
Display advertising is represented by a wide range of different formats and contains items such as text, images, video, and audio. Real-time bidding refers to the buying and selling of online ad impressions through real-time auctions that occur in the time it takes a webpage to load. Those auctions are often facilitated by ad exchanges or supply-side platforms.
The CPA (cost per acquisition) represents the amount of money you have to spend to achieve a conversion. In an AdWords campaign, it is determined by two factors : the CPC (cost per click) and the conversion rate. Thus, optimizing your CPA will consist in analysing these elements. Note that you can set up your conversion goals in AdWords and thus get accurate cost per aquisition statistics.