I'm generally the outspoken one in my group of friends on matters relating to sexual health. I yell at my male friends for being so stubborn about disliking condoms. I have thus far managed to avoid any STIs or unwanted pregnancies (and for me, any pregnancy is one I don't want).
Right now, I'm in a monogamous relationship where we use condoms every single time we have sex. If I found out I had an STI, I would have to tell my boyfriend to his face. However, if I had multiple casual partners and found out that I may have infected one or more of them, I'd like to think that I would let them know about their new "pet" over the phone or in person. It seems more honorable that way.
Having said that, I think the e-card is a really good idea, because if you were too mortified to let them know who it was that infected them, at least they could be alerted anonymously and take steps to halt the spread of the infection.