Engagements. Users who have reacted with, commented on, or swiped through an ad (if using an image carousel or other multimedia placement) can be retargeted.

Website traffic. If users visit your business website and you have installed the Facebook pixel into your header code (there are loads of tutorials on this), you can get back in front of them through retargeting.

Email list uploads. You can upload a list of subscribers to Facebook for retargeting purposes. Facebook attempts to match email addresses to users. The success rate is wide here, ranging from 40 to 80%, but may be a smart strategy for you.