I don't belong to Facebook. Two acquaintances I know do. They discovered they both knew me and have been e-mailing each other about me.
I discovered this when one person forwarded all the other person's e-mails about me to me. I was really dismayed and upset. It has caused a breach in both friendships. I don't feel I can ever feel comfortable again with them. Is this something that can be avoided?
Unfortunately, your dilemma is more basic than Facebook or other social networking sites and cannot be avoided so long as people choose to talk behind other people's backs.
Long before computers were a means of communication, people did things like pass handwritten notes between each other, notes filled with the same hurtful comments that your friends articulated on Facebook. Regardless of the means by which the catty comments are conveyed, communicating them in any form is rude, hurtful, and wrong.
It's galling that it happens not just in people's personal lives, but also in business. One of the basic tenets of communication that I teach in my business etiquette seminars is not to use a public form of communication for anything that you don't want other people to see. Think of it this way: If you can't post the message on a bulletin board for anyone to see, then don't use social networking vehicles - or e-mail, texting, voice mail, or even handwritten notes - to convey the message. At the moment you most don't want it seen, it will be seen by the person you don't want to see it.
In your situation the damage is done. The hurt and betrayal by both parties can't be erased. You have two choices: To engage either or both of the people in an attempt to clear the air and begin rebuilding the relationships; or accept that these people were not the people you thought they were and realize you're better off not associating with them anymore. It's a tough choice.