There are so many ways: - you opened a web page with a facebook's like button and facebook analyzed your browse history. If you browse for something in google

There are so many ways: - you opened a web page with a facebook's like button and facebook analyzed your browse history. If you browse for something in google shopping you might want to buy it, so facebook added it to your interests and showed an ad later to you. (Probably this was the cause) - google and facebook knows your location because you have a google/facebook app in your mobile. It is easy to know where you work and where do you live. Even without GPS, as they can use the wifi public IP to know your location (among other things) - your browse makes a fingerprint unique to you, even if you are not logged into any platform or you don't have an account on them they know you are you. So they can collect data. If you later login into facebook/google they match the fingerprint with your user. They can also use several fingerprint, because you use several devices. I think this is called supercookies. There are companies that just do this. - you have two different devices with two different accounts, but you have the phone number of the other in your contact list and sometimes you have both devices with you. Facebook/google will suppose you both share same interests and show ads that match one of your devices to the other. - "do not track" is a suggestion, some people ignore it because they really want to track you. - google and facebook probably use other ad networks to sever ads, so facebook probably asked google (or another ad network) for an ad for you. You probably shared your phone number / email in both facebook and google. - if you want to be fancy, machine learning could also help to target ads for you, but I think the other ways are so easy to do and work so well that you don't need complex things to show ads, just track the user.