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Sharing or not sharing data with another app is likewise not an unusual decision. FB is not a public utility"
[the following paragraph does not assert that FB is a monopoly, it is agnostic]
The law says that a company with a monopoly needs to act a bit like a public utility. Recall what Microsoft did to Netscape -- was there anything wrong with that? Is a company allowed to take action against a competitor? The normal answer is "Yes". But different rules apply if the government can prove that a company has monopoly power in some market. Actions that are allowed among normal competitors are no longer allowed once a company has near monopoly powers. I think fast growing tech companies get into trouble because the founders doesn't realize how much the company has grown. Bill Gates could use aggressive tactics during the 1980s because Microsoft was still small. He got into trouble in 1994 because he didn't realize how much Microsoft had grown, and how much the rest of the world suddenly regarded it as a behemoth. Therefore he was no longer allowed to pursue the aggressive tactics that he'd previously been free to use.
Arguably, the same reality has now caught up with FaceBook.