The most annoying people in existence and usually leave you guilt-ridden when you get rid of them. Usually are people who you have no interest in gaining any form of relationship with, but one friendly remark and you have a new buddy.
When you meet gregarious friends of a friend of yours and they automatically become your friends, too. While there's potential for meaningful relationships, instafriends usually remain cheerfully superficial.
"I went to a party with Frank, he introduced me to Sally, Joe, and Bob. We all exchanged numbers, yay, three instafriends!"