Because you can use those people that have taken an action with your Page (liking, commenting, sharing) and make them an audience for your ad. So Facebook is willing to make this a cheaper ad because you are retargeting people that have connected with you.
Think of it this way. Getting numbers to cold call for marketing is usually way more expensive than just calling the people that gave you their number. So basically, it sucks to be the new Page trying to build your Likes and following.
Who is my audience, specifically?
Key here is specifically. Unless you have a great list of email addresses customers have given you, it can take some time to find the specific audience on Facebook you want to show ads to.
If you have a generic summative of your customer, build that first. Then copy it to get specific for each instance you would want. After all, you don’t have to send an ad to everyone. Make ads that appeal to what that specific audience wants.
Making a specific audience in Facebook Ads is part psychology part sociology. What I mean is that you need to know where you audience is, likes, dislikes, demographics, The best people at making Facebook audiences are not educated as marketers. They have backgrounds in fields more relative to psychology, sociology, and public relations.
You have options to choose items the audience fits at least one of the characteristics, items that the audiences does not fit, and items that all of the audience has to fit. Think of them as Boolean search tools (or, not, and). So, you can see that there are a lot of options to identify your audience.
Do I have a list of emails I can build my target audience off of?
Good for you! You now are much better off than most. Take your list of emails you got and upload them into a custom audience. But you are not done. This is a starting point. You still have a lot of work to do. Just because someone gave you an email address does not mean they are automatically a contact in Facebook. The email has to be the one used by a Facebook profile to work.
Then, go through applying the tools to specify the audience to find who in the list of emails is a good fit for your ad. It might only be a fraction of them. I have seen numbers as high as 80% of those that gave their emails and as low as 20%. It partially depends on how you got their emails. But you aren't done there.
Having a custom audience allows you to then make a "like audience". Which has Facebook find interests and demographics in your email list to create an audience that are similar matches. Once again, how you made your email list makes this more valuable.
Using Facebook Ads to build an email list
Now, you might be saying, "I don't have an email list. This sucks." And you are partially right. But Facebook makes it relatively easy to get started on building an email list. It is using its ad feature Lead Ads. Which basically is a signup screen in Facebook.
You do have to make an audience to target this ad at to get your emails. But if you are starting from scratch, this isn't a bad way to go. And don't ask for a lot of info. If you are interested in getting their email address, just ask for that. Easier for a person to be ok with sharing limited info.
So, someone engaging with you on Facebook is good for targeting an audience, but someone's email address is better.
Using Facebook Pixels
Odds are you have a website. Hopefully. If you don't, stop right here and go build one... Welcome back.
Facebook pixels allow you to track and then retarget people that visited your website. The settings in pixels can make them very powerful, especially for e-commerce sites. Ever wonder how you starting seeing ads for things you looked at on a website the next day you went to Facebook? This is how. Similar in concept to what people have been doing with Google Ads for years now.
You can use pixels pretty easy if you just want to build an audience of people that have visited your website. Facebook will give you a group of code to just put into the header of your website. Probably where you put the code to track visitors like Google Analytics. If you want much more than that it will take some time, and possibly software, to do so. And you might have to hire someone.
If you have a business with a customer base already. Save yourself a ton of time and money and get their emails.
Talking to your customers and truly understanding who are your customers is always worth it, even more so with Facebook Ads.
If you don't know how to build your audience right, get someone to help you.
Hopefully you are now set on the right path to build a better audience in Facebook to target your ads. Enjoy and good luck!