Sure, they retarget ads they think you might want to see, but the difference is when using Google you are actively looking for something, not passively.

Sure but I still don't think there is much meaningful difference.

Google looks at both your current search/browse activity and gathered data (the gathered data being especially important for all the very many Google-served Ads off of google.com)

Facebook also looks at your current search/browse activity and gathered data.

The only real difference is Facebook search is universally entirely worthless so far less of that signal gets used relative to more passive snooping.