So, two nights ago my wife is perusing the site of a business she is thinking of making a purchase from. She was using our home computer and she was not using facebook. Neither she nor I had ever looked at this site before. The next day I go to work. I am on my work computer. During my afternoon break time, I check facebook. The first thing that pops up is an ad from that same business. I have never seen an ad from that business before. This is too much of a coincidence. So, somehow, FB got wind of my wife looking up this site (again, without facebook), on our home computer, associated me at work with me at home, and then pushed an ad from that site. How did this happen? Is there any way that I can stop it?
Did you ever used Facebook on that browser?
If this is the case then when you visited the website, Facebook was able with a tracing cookie to pickup your visit, store the data (timestamp, ip...) from the visit in his big database and use it to present relevant ads based on your tracking history.
It happens with google and way more pervasive. Did you ever noticed that google ads are following you from site to site, device to device?
And the best thing? When you (and me of course, we all in the end) created the account we agreed on this accepting the Terms and Conditions.
Many websites contain a Facebook 'like' button that is also used to track users who are not logged into Facebook. Using all the information collected, it's not difficult to make the connection between a logged out user and Facebook users.
This behaviour was in the news last year and received some attention as people were not expected to be tracked by Facebook when they were logged out. Also, some people are not happy to be tracked by Facebook when they are actively trying to avoid sharing information with that company.
Facebook and advertising companies in general are very good at collecting information and putting data together, even if it was anonimised.