It stands for "Best BlackBerry Messenger Buddy For Life"
1) Your best friend on BBM
2) Your best friend (who also happens to be on BBM)
2) usually a joke phrase, used to laugh, a little, at geeks. And people who use phrases like BBFL.
BBM friend 1: oh cool now we are bbm buds
BBM friend 2: does that make us bbbmbfl
BBM friend 1: haha yeah that sounds cool
BBM friend 2: I was quite pleased with that