They weren't on Facebook's website at all. They were using an online pharmacy's website, and Facebook gets sent information about this because the site embeds F

They weren't on Facebook's website at all. They were using an online pharmacy's website, and Facebook gets sent information about this because the site embeds Facebook's tracking scripts. This is one of the insidious things about adtech: they track you all over the internet, not just on the adtech companies' own sites.