My first Postcrossing card!

I received my first Postcrossing card today! Whoopee! It's from Finland. I have sent out four cards now myself: one to Germany; two to Finland and one to Kansas, USA. Unfortunately I have realised today (on receiving my card) that I forgot to put the identity numbers on the last three cards I sent. Duuuh! This means that the people receiving them have no means of logging them on the site, which means I can't get more addresses because it'll look like I've still got cards 'travelling! I will have to send them each another postcard.