…is rather unlikely (although MySpace is testing this theory)
But if it does happen, at least you have the email addresses of all your members to invite them to a new platform.
…unless you’re using Facebook groups.
Facebook groups aren’t likely to experience a permanent platform failure, but isn’t reducing organic reach to 0.1% the same thing? This isn’t happening overnight, it’s happening gradually (already).
But just because you’re being boiled slowly doesn’t mean you’re not still getting cooked.
Worse yet, it’s extremely hard to mitigate against once it’s happened. The best mitigation strategy is to move when your reach is still high enough to tell people you’re moving.