Two ways:

1. Facebook runs its own ad network, so you might see Facebook ads even if you never go to Facebook's website.

2. You might still go to their website even if you don't have a Facebook account. Many local restaurants and community organizations only have a Web presence through their Facebook pages.

Facebook Ads on third party websites.

https://techcrunch.com/2016/05/26/facebook-starts-selling-of...

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.

From the post: "Facebook ads and measurement tools, which enable websites and apps to show ads from Facebook advertisers, to run their own ads on Facebook or elsewhere, and to understand the effectiveness of their ads."

By having data about you facebook can infer stuff about other people who are on Facebook and serve better ads to them.