1. Verb - the act of removing someone from your friends list on any social network.
1. a. Verb - the act of removing someone from your category of friends generally, not limited to removing them as your friend on any social network.
2. Noun - someone who used to be your friend, but you no longer consider them to be a friend, for any number or variety of reasons.
After Dave posted private photos of her wild sweet 16 birthday party, featuring much nudity and general craziness, Tilly thought he's just gone too far. So, she unfriended him on myspace, took him out of her buddies list, and deleted him from her phone contacts.
Sookie asked her a couple weeks later about it, "What, I thought you two were tight?" Tilly shrugged. "Well, we used to be bffs but now we're unfriends and that's just how it is."