What are mutual friends on Facebook?

Facebook activity privacy. friends see what i write or like on another (non mutual) friend's wall.?

• On Facebook friends can see what i post to another friend's wall or that i liked a pick on their wall even though the friends are not mutual friends. how do i stop?

• Answer:

  It actually depends on the privacy settings of that person. They control who sees what is posted on their wall, it's most likely set to friends of friends or public. That means any of your friends can see their wall if it is set to friends of friends. Same with photos, a lot of the ones that get shared are shared as public, so anyone can see it if they look on that person's profile. Essentially you only control who sees what you and others post on your own wall, if you don't want other people seeing it, private messaging is the best way. Or post on your wall, set it to only you and that person, tag them, then only you, them and their friends will see it.