Facebook runs an ad network (Facebook Audience - how successful, or how much used this is, I don't know though. If you're visiting a site or using a mobile app

Facebook runs an ad network (Facebook Audience - how successful, or how much used this is, I don't know though. If you're visiting a site or using a mobile app served from facebooks ad network, you don't need to be on facebook or have a facebook account to be shown an ad.