Heidi - I send postcards both ways - sometimes I put several in an envelope and sometimes just mail one at a time. They seem to make it okay and are mostly readable - I have gotten responses that indicate they have read something on a card I sent.
Sometimes I send blank ones so they can write home on them as well. I have gotten a few self-address postcards back from contacts that have survived the trip well (okay maybe they were a little dusty from the sand, but I could still read them!).
Hope this helps!
Bye the way, I scored a huge packet of postcards at the thrift store today for 95 cents. I saw some TX, NY, Taos NM, Las Vegas, and Disneyland in the package so I will send those out to the contacts.