You’re probably running prospecting and retargeting campaigns on Facebook. But there’s a third campaign type that many companies surprisingly forget: remarketing.

First of all, what is remarketing? It’s marketing to former customers. Here’s the taxonomy for the three types of campaigns.

Prospecting: marketing to consumers who have never visited your site, at least according to the platform. Retargeting: marketing to consumers who have visited your site but have not purchased Remarketing: marketing to former customers.

What’s the benefit of remarketing? Well, the people who are most likely to buy from your site are people who’ve bought from your site before. Remarketing is worth it because it allows you to get in front of the people who are most likely to buy. What’s not to like?

Well, the objection I usually hear against running remarketing campaigns is it’s a waste of money because, after you acquire a customer, you have their email address and, sometimes, phone number. In other words, you can remarket to customers for free and don’t have to pay Facebook to reach them again.

Fair enough. But email is not a panacea. Even the best email campaigns have open rates less than 25%–and often much less. Not to mention the customers who have totally unsubscribed. Remarketing allows you to get in front of the many customers who, let’s be honest, don’t read your emails.

Still, you need to be careful. Remarketing campaigns have the potential to be significantly not incremental. Your existing customers are by far the most likely group–at least compared with unaware consumers–to buy from your brand. Remarketing campaigns will sometimes only pull forward purchases that would have happened soon anyway or, worse, take credit for purchases that would have happened at exactly the same time (view through conversions).

So your ROAS or CPA targets on remarketing campaigns should account for this and be way higher than your other campaigns. How much higher? I like to segment the remarketing campaigns out. If they’ve purchased in the last 3 months, say, the ROAS targets should be much higher (because additional purchases are less likely to be incremental). If they haven’t purchased in a year, ROAS targets can approach those of your other campaigns.

Also, don’t expect to be able to spend that much on remarketing campaigns efficiently. Unless you’re Starbucks, the universe of people who haven’t been your customers is much greater than the universe of people who have. So the prospecting and even retargeting audience size is going to be orders of magnitude larger than the potential remarketing audience.

If you’re not doing it already, give remarketing a try. You should be able to reactivate some lapsed customers you would have fully lost otherwise.