You are right that "we use (would+have) when talking about imagination or supposition as in if rule...(if had...would have)" However, here "would have" is used to refer back to a time in the past from a point of view in the future. This sentence means the same as, That includes posts you shared with friends of friends too."

thanks gray

I think Ariana is right. This seems to me like an instance of using "would have" for a hypothetical situation or a supposition. The meaning is that certain types of posts "would have" been shared with friends of friends IF you'd made those types of posts (it's hypothetical because Facebook doesn't know whether or not you've actually ever made any posts of this kind). I also agree with Ariana that this isn't a very good example of how hypothetical "would" is normally used. A phrase like "may have shared" would be more appropriate in this context.

rhanks so much ariana

Good questions, Hazem! It's true, "would + have + past participle" is used for hypothetical past situations i.e. they didn't really happen. For example: "I would have moved to Europe if I had gotten the chance!" In this situation, it's used in a more abstract way. In fact, it's not even the best use of English grammar, in my opinion. Nevertheless, it means "may have," like, "Maybe you shared posts with friends of friends. Maybe you didn't. However, if you DID, they will become private." In this situation, it's possible that it DID happen.