I again think most “lost” are mostly not registered, or not sent.
And then of course some also truly lost.
Some people are oh yes, I want mail, they sign, and ask, when do I start getting mail? not thinking they need to send first. And when they look where they find cards, maybe they don’t, or when they buy stamps, they consider it too expensive. So maybe they already have the five addresses, and then stop this hobby without sending.
And sadly, some people pick who they send to.
Before I joined, I read a forum where people openly told they won’t send to a new member, because they are likely to not continue, or don’t register. They told not to send, if someone was not online.
( But, this behaviour can’t continue long, which is good.) They didn’t register, if the card was not good enough, or they waited if the person sends a second, “better” card, or they didn’t register to “teach” that person. So when I joined, I was worried will I get a card, because of course I was a new member.
Some think if you open many accounts, click the addresses, you will get these cards too, not knowing they need to be registered.
This is also based on all my other mail, that hardly never is lost.
The “lost mail rate” here is unnormally high, so therefore I believe most are not just registered (either forgetting, not knowing how, not wanting etc.) and some mail not sent (they join, then find it too much work to look for stamps, cards, write it…or they pick the profiles where they send to, forget, use too little postage).
When I think it can be a whole year in my other mails, and nothing is lost, and here I now have 15 expired (yes, some will still arrive), it seems unlikely only Postcrossing mail is lost, and all other mail travelles better.