Good news. Like a house key, the FIDO keys are basically anonymous. So, as long as you didn't e.g. leave it with your wallet on a bus, or write on it with non-erasable marker "simongr3dal@example.com" losing the key is not that scary.
In fact, unlike a house key, the Security Key works fine for its new owner, they can register it to sign into their Facebook, or whatever, that will work fine. Facebook will have no idea it's your key, now it's their key.
If they know you're simongr3dal@example.com on Facebook then that's a problem, yes, as obviously they can sign in as you, but if it's so hard for you to remember what you signed up to, seems like it'll be pretty hard for a hypothetical finder to figure out too... "Hmm, I wonder if this random stranger was into Diaper Porn and Antique furniture?"
In fact, unlike a house key, the Security Key works fine for its new owner, they can register it to sign into their Facebook, or whatever, that will work fine. Facebook will have no idea it's your key, now it's their key.
If they know you're simongr3dal@example.com on Facebook then that's a problem, yes, as obviously they can sign in as you, but if it's so hard for you to remember what you signed up to, seems like it'll be pretty hard for a hypothetical finder to figure out too... "Hmm, I wonder if this random stranger was into Diaper Porn and Antique furniture?"