Can I send workflow events to Facebook to build custom audiences?

Yes. Add the Facebook Conversions API step to a workflow and send events to Facebook to build a custom audience. For example, when a contact replies that they are interested in getting their HVAC serviced, send that event so they see your Facebook ads. Or do the reverse and send the people who did not respond, so they still see you on Facebook. You can also use these audiences to build lookalike audiences.