"use on a dating app" 안돼요. (incorrect ❌) "use a dating app" 맞아요. (correct ✅) -> "Why use a dating app if you already have a girlfriend?"

Same here, I've never seen that version with "on", the natural way would be just "use a dating app". I'm not a native speaker, but I've been fluent for many years and I'm a professional English teacher as well.

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