What was the last thing you searched online? Do you remember? Facebook does.
Facebook keeps track of your offline activity to show you relevant ads. As frustrating as that sounds, it can be a helpful tool. It can help find products you had no idea existed or that you need and guide you through a purchase.
Have you started a new hobby? Facebook wants to show you relevant purchases to help make your foray into that hobby a success. New parents can be shown recommendations of products that will help them navigate the ups and downs of their newly changed life.
Tracking activity sounds like a technological nightmare, but when you look at it in terms of how it can enhance your life, it opens up possibilities that you never knew existed. Give it a chance and if you still don't feel comfortable, you can change your offline tracking by clicking settings - settings and privacy - settings - off Facebook activity.