Most of the time, when you advertise it's because you want to increase your sales. Obviously increasing your sales is the desired end result for most business activities. However, some activities can help your company while not directly increasing sales instantly. Take Facebook ads for instance.
Facebook ads should not be considered the "bread winner" of your marketing activities. They are most likely not going to produce a lot of sales. However, they give you the opportunity to easily reach your specific target market. When you create a Facebook Ad, you can pick keywords and phrases that people have as interests in their profiles, and then your ad will appear when that person is cruising Facebook. You can narrow your reach down even more by age, marital status, location, employment, etc. Very targeted! And the more targeted, the better, because you do not want to waste your marketing efforts on those that are not in your target audience.
When creating your Facebook ad, it's important that you convey the most valuable bit of information in the display. This is because not very many people click on the ad. I know... seems silly to purchase a Facebook ad if no one clicks! But, as I said before, your Facebook ad should not be for sales alone. Use it as a branding mechanism, and also as a way to develop a relationship with your target audience. While people are browsing on Facebook, they will at least see your ad. Maybe they've heard of you before, and they think to themselves "oh there's that guy again, he's everywhere!". Lucky you! It takes a person seeing an ad many times before they decide to purchase. Your ad is a part of the marketing cycle.
Another quick tip for Facebook ads: try testing out different ads every few weeks or so. Another great feature of Facebook ads is that it allows you to track the results of your ads. You can see when people clicked, how many, etc. See what works and what doesn't! The ads are quite cheap, so it's not difficult to test.
Lastly, if you are trying to sell a product through your ads, set up a unique affiliate link. That way, you will know which purchases came from Facebook ads.