It's a person that's kept around for a period of time because they are of some sort of benefit to the person that's pretending to be their new BFF. The type of person that keeps Flavor of the Month Friends is egotistical, always cash strapped, borrows money and never gives you the money back, cannot be trusted with any personal 411 since they will tell it all over town, creeps on the significant others of their friends/flavor of month friends, a fickle fake "friend" who will uses you to benefit themselves and in the end they don't recognize who their true friends are because they just met their "new BFF" 24 hours ago.
Girl 1: Sheesh, I can't belive Sophia has a new Flavor of the Month Friend again. She told me that that she met this girl yesterday at the bar and now they're BFF's already.
Girl 2: I can't believe she ignores her real friends as soon as she finds a new "BFF" .I guess that's now her new "bestest girl"!
Girl 1: I think it's time to drop that girl because she don't recognize her real friends.
Girl 2: Yea, I agree. We're done with that girl!