You have to get this part right, and there’s no one way to do it. Here are a few methods that have seen successes.
1. Upload your existing customer list and create a lookalike audience using the Facebook ad tool. (really good resource on setting up custom audiences). This method works through Facebook’s ability to find people who are similar to your existing customers. You are relying on their ability to find people similar to your existing audience, but since Facebook does have a wealth of knowledge, I’ve heard this can be very effective.
2. Target people who “like” your partners. For example, if you are a marketing company and you know that a lot of your best customers use another marketing software (that compliments yours, doesn’t compete), then you can target people who “like” your partner.
3. Target Interests your customers have. Say for example your audience is entrepreneurs. A lot of them might be in startup groups on Facebook or have attended Startup Events in the past. You can target those audiences through Facebook and go to your audience where they already reside.