Yes, it seems that there are people that join just to try it but never completing sending the postcard. It is a situation found in all clubs. They are just courious about the project and they don't continiue with the sending.
For situations like this, you have the option "Not received". Postcards in this situation will expire in 90 days and will appear in the first page after you login. Those expired letters have another 90 days to be resolved. If you mark the postcard as "Not received" and the sender does not login again, then the postcard will automatically be deleted and you receive the slot back.