The worst thing I've done on Facebook was to call someone out on what I perceived as bad behavior. I've always hated Facebook birthday greetings because I think they are lazy and impersonal. For example, on my 40th birthday, a dear friend put a quick "Happy Birthday" on my wall, when this relationship qualified for a phone call at the very least. Facebook has lowered the bar for something that used to be so meaningful.
My best friend died in November. Her birthday came around in December. I shouldn't have done this, but I looked at her Facebook wall that day, and among the many expressions of sorrow and loss, there were indeed messages from people who clearly did not know she had passed. One such message was from a mutual friend of ours from college. I was enraged. How could this person not at least click through to her w all to see what was going on in her life before blithely clicking that easy little button that allows you to say "Happy birthday! Hope you're well!" and then wash his hands of her and move on?
I sent him a message. "Hey, did you know our friend died?! SHE DIED. You might want to check people's Facebook pages before you post things without thinking."
Of course this guy was devastated and embarrassed. Later I felt terrible for doing that to him, realizing I had acted out of grief and hurt, and I apologized. I never heard from him again.
And we're not friends on Facebook.