Keep in mind that at least the FTC generally considers a market to be, roughly, goods and services that are close substitutes for one another[1]. That is, if one good or service can be substituted for another, those may be in the same market. However, as the FTC's summary says, "evidence that customers highly value certain product attributes may limit their willingness to substitute other products." The size and breadth of a network is a product attribute that affects a customer's willingness and ability to substitute any other product.
For anyone else into this topic, "Antitrust and Social Networking" (2012) is a good place to start: https://lawecommons.luc.edu/cgi/viewcontent.cgi?article=1188... (PDF).
[1]: https://www.ftc.gov/advice-guidance/competition-guidance/gui...
There may be something in newer legislation which has made data portability a priority. And Apple has just changed its practice, but due only to very specific pressure.
So I’d say this is rather pathetic, but not illegal.