The parent asked how do you define at all. What I wrote is just the dumbest way which demonstrates how it can be done. This dumb solution holds up even in your scenario because new media doesn't gain significant user base every other year. If the list is outdated, containing Facebook and Instagram alone, that would still capture a huge part of the problem already. You can probably figure a slightly less dumb alternative that wouldn't require amendments just to add another platform. Folks talking about the impossibility of defining something or implementing something in law often ignore obvious solutions, existing working processes, and present this false dichotomy of a perfect solution vs impossible to solve. Sometimes it's a matter of ignorance, other times it's driven by (conscious or subconscious) libertarian beliefs.