When someone who is new to a social circle ends up becoming more important and loved in that group than the person who introduced them to said circle. Often times, the 'introducer' actually ends up getting kicked out of the group through a gradual phasing out. Most times friend jacking is the result of some inherent character flaw of the person who has been kicked out of the group.
Nikki introduced Nelson to Chloe and Cathy two months ago. Now they're BFF with Nelson but don't even talk to Nikki anymore. She got friend jacked.