Setup Conversion Leads API integration using Salesforce APEX triggers on Lead objects
This PDF guide explains how to setup a trigger on your Lead object in Salesforce. Whenever the lead object is created or updated, and it came from a Meta lead, an API call will be sent to Meta with information about the Facebook Lead ID, and the status/stage of the lead.
For more about conversion leads, see: https://developers.facebook.com/docs/marketing-api/conversions-api/conversion-leads-integration
The guide can be found here.