Personalized advertisements exist because all-sites exist.
Facebook isn’t a pets/cars/vexillology site, it’s an all-site; it and its kin are designed to contain almost all of humanity’s interests. It stands to reason that all manner of ads would fit in there, and in order to make these ads useful you need to filter or target them. If you see the history of targeted advertising, a case could be made that Facebook invented it as we know it today. No diss meant to Google and Googlers that might read this, I’m sure they played a big role as well.
All-sites still have a context in which their ads appear. You are looking at some content on their site, so thy can advertise in the context of that content.
Facebook isn’t a pets/cars/vexillology site, it’s an all-site; it and its kin are designed to contain almost all of humanity’s interests. It stands to reason that all manner of ads would fit in there, and in order to make these ads useful you need to filter or target them. If you see the history of targeted advertising, a case could be made that Facebook invented it as we know it today. No diss meant to Google and Googlers that might read this, I’m sure they played a big role as well.