You don’t have to explain why you’re not on social media.

Not being somewhere does not require a statement, though it tends to get treated as one. Say you are not on an app and people hear a position about the app, then ask you to defend it. There is no position, there is just a preference about how you spend an evening. Absence needs no manifesto.

If it comes up, keep the answer short and slightly bored: it is not really my thing. Do not offer the reasons, because the reasons are what get argued with. Then ask them something else and the subject closes on its own.