1

If I posted a photo on a friend's timeline, who is not a mutual to my other friends. Who can see this post?

My friends and their friends or their friends only .

3

If you post directly on their timeline, they control the privacy setting. They have just one setting for all posts that others post on their timeline. If they have it set to Friends, then only their friends will be able to see it. If they have it set to Friends of Friends or Public then your friends (and all other friends of friends) will be able to see it as well.

You can check the privacy icon on other posts from others on their Timeline to see how they have it set, since there is only one setting for all posts from others. However, keep in mind that they could change the setting at any time and that will affect existing posts.

An alternative is to post on your own timeline and tag them. This will give you more control over the privacy setting. If you post to your friends then you can choose whether to make it visible to the friends of anyone that is tagged as well (in the Custom dialog), and can exclude some of your friends if you don't want all of your friends to see it.

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