My sister has been visiting for a few weeks.
On several occasions, she has received notifications for “suggested groups” and ads on Facebook based on searches that I have made. I am not on Facebook but was several years ago.
Yesterday, I was looking for takeout food on my phone, now she is being asked to join a take out and delivery group for my town.
Previously, I was looking for refrigerators and she got Facebook ads for refrigerators.
I can understand that the internet knows she is in this town and not at her home but how does Facebook know what my phone is doing especially since she has a different last name.
Also, why would Facebook think that she would be interested in refrigerators just because I am?
The way internet in your house typically works is that the router has an IP (internet protocol) address that the rest of the world sees. All traffic coming from your house has this IP address and any site that anyone visits will show as coming from that single IP address. But inside your house, each device has a unique local IP address that your router manages. The router manages the internal and external traffic so that all the internal devices can properly send and receive to the internet.
For example, your router might have the IP address 9.22.33.44. Any site that is visited from any device within your house will look like it’s coming from 9.22.33.44. So facebook, amazon, etc. all think all traffic is coming from 9.22.33.44. But inside your house, your router is giving each device a unique address like 192.168.56.1, 192.168.56.2, etc. Your phone has the .1 address, her phone has the .2 address, and so on. Your router is managing all the traffic inside and outside. When you or your sister visit facebook, facebook sees the traffic as coming from the external 9.22.33.44 address.
So anyway, sites like facebook might be unifying suggestions based on traffic coming from that external address of your router (9.22.33.44). They may figure that if one person in the household likes group X, maybe other people in the household also like group X. This may make sense for things like the refrigerator example. If you are looking for a fridge, then likely everyone in the household may also be looking for a fridge.
Obviously there could be problems with things like this. If the people in the house are just roommates, they may not have the same interests at all. And the same thing with a workplace, school, etc. Just because all the traffic is coming from a single external IP address doesn’t mean that everyone at that site shares interests.