IBF is an acronym for "Imaginary Best Friend". It is used in reference to a celebrity or high profile person who one likes to fantasize about being a friend. It's similar to celebrity crush in the imaginary aspect, but IBF is not a romantic fantasy, it's about friendship.
Reese Witherspoon is my IBF (Imaginary Best Friend), and right at this moment, she is within a five-mile radius of me filming a movie. (From a story called "Pieces of Reese" at NBC4 Washington website.)