A contact added to the audience you select, or the existing one updated if that email is already subscribed, so a repeat enquiry never creates a duplicate. Answers are written to that audience's real merge fields, FNAME and LNAME included.
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 you can see which ad is actually growing the list, not just that a subscriber appeared. Every lead is also scored before delivery, with real checks on the email address, so an obviously fake one never gets the chance to bounce and hurt your sender reputation.