As you say, the data domains of different services may simply be logically incompatible, which is fair enough. As critical as I am of social media platforms, I

As you say, the data domains of different services may simply be logically incompatible, which is fair enough. As critical as I am of social media platforms, I don't think they should have any obligation to implement the abstract ability to "import" data in general. As long as they provide their users with the ability to export their data in a reasonable fashion, in a well-defined and consistent open format, and as parseable as technically feasible, I believe they have done their duty. What's important is that a Twitter, or Facebook, or Reddit alternative should be able to implement data import from their respective competitor, without facing intentional anti-competitive difficulties. Let "the market" decide which services want to implement the import of which kind of data.