If you’re on Facebook you should make yourself familiar with Instant Personalisation.
Facebook will share the data you make public with their business partners so when you visit their web sites they will display details based on your information. If any of your friends have ‘liked’ the site you will see that. If they have left a comment you will see that.
If you don’t want to share your information with web sites so you can see what your Facebook friends are up to you can switch the service off inside of Facebook, or you can switch it off at site you do not wish to share with.
If you make all your information private this also prevents Instant Personalisation from sharing your info