Independent of Postcrossing, I sought pen pals on Facebook and a few other online sources. Many networks were free but the "pro" column ended there; in the "cons," their layout and design were at least unattractive and sometimes unnavigable, and it was very difficult to compel a real person to correspond. Many accounts had been abandoned due to lack of interest or perhaps being plagued with spam: setting up an account on these pen pal networks was an invitation for a barrage of "God bless, I'm a refugee from [west African nation] being held in [another west African nation], and I'm looking for love," or "God bless, I'm yet another nephew of this petty warlord you've doubtlessly heard all about, and I need your bank account info to send you ungodly amounts of money."
There's also a pen pal Usenet group in existence--alt.penpals--or it's only called that these days: currently it's overrun with obnoxious high school kids from the States (plus one middle-aged man who is only comfortable associating with teenagers), using it for a private message board. Lacking sufficient Web-savvy to figure out how to set up their own free forum in Google Groups (or any of a hundred other programs), they insult and harass anyone who shows up genuinely looking for a pen pal. More's the pity, it also attracts its share of sexually deprived subscribers from third-world nations. Don't go there expecting anything good.
None of this happened in Facebook. I found a group dedicated to exchanging postcards and made the acquaintance of a gentleman in Genoa who takes great pride in sending postcards from various Italian cities. His hobby is to collect postcards not only from different countries, but from different regions within those countries and cities within those regions! He has hundreds upon hundreds of cards and categorizes them diligently on his Web site--it is an impressive collection indeed. Below is one of the cards I have from him. Thanks, Davide!