Only on facebook, not in real life. Even on facebook, I sent the person a message, found out why they'd unfriended me and am now friends with them again.
In regular life, I only have a small handful of close friends and we've all been intensely loyal over many years and lots of craziness and hardship.
I did dump one friend years ago because he lied about me in an unsuccessful attempt to steal my girlfriend at the time. The saddest part was that I was getting ready to dump the girlfriend anyway and if he'd waited a couple of weeks I'd have encouraged them to get together.