Create Targeted Digital Advertising Audiences with Advertising Studio digital marketing ads

This is really helpful, But there is one thing i was unable to understand what if initially i do not have the contacts or subscribers in my MC data base.

How we are going to capture the new audiences from my Facebook Advertising campaign ?

Similarly my company using Facebook business and Pardot for marketing, Facebook provide us the option for targeting the audience either it can be our (1.)followers (Existing data for our account) or (2.)completely new audiences (filter by demographic, gender, etc).

So my question is how I can target the new audience(refer 2) in advertising studio ?