Facebook enables advertisers to target current customers and also allows you to create lists of audiences that share similar interests to your customers.
It is easier to sell to current customers than to look for new ones, so if you have a database of your customers, you can strengthen your existing connections by using Custom Audiences.
Facebook will cross check your data against their data to help you find the people you know, on Facebook. The only data you need is email or phone numbers from your customers; however, name and surname helps Facebook to find them on their platform.
This information is encrypted within Filed and Facebook to protect customer relationships and avoid data breaches.
From this list of customer you can also create Lookalike Audiences. Facebook recognises your customers, and tries to find users with similar interests and behaviours. You can choose from a 1-10% similarity level of audiences.