The many ways (strange/unusual/paranormal/hallmark/cheesy/corny/sweet/loving/controlling) your BFF behaves towards you and your shared friendship of devotion, adherence, usage, characteristic ,state or condition; which designates a doctrine to your referenced designated BFF.
-ism suffix (Greek) - formation of nouns which denotes practice or action from verbs.
Janny: Car - you need to get off your whorish way's on FB!
Car: Janny, thanks for your bffism all these years! Without your bffism, I would not be the same.
Janny: I TOLD YOU! STOP BEING A SLUT!
Car: Awwww - I heart you too! by the way, your bffism stinks!