A Lead record on your org, with form answers mapped to standard and custom Lead fields, so assignment rules and reports treat it like any other inbound lead. If the same person submits again, their open Lead is updated instead of duplicated.
Alongside the answers themselves, each lead carries the campaign and ad it came from, the Facebook Page name and the time it was submitted, so your attribution reporting still means something once the contact is inside it. Every lead is also scored before delivery, with real checks on the email address and phone number, so an obviously junk submission is flagged before it clutters your pipeline.