For me personally, I do not use "FYI" often anymore, as some people consider it to sound rude or belittling. As for someone to say, "Your concert was off the hook", it would mean that the concert was very great and Fun, indicating that you did well! :)

FYI and FWIW, in my social and work circle (large software firm in the Seattle area), people say "FYI" out loud (as well as writing it) all the time. It's as often as not in the form "Hey, just FYI, I'll be OOF tomorrow". They pronounce "OOF" as well (out of facility, meaning out of the office). "Off the hook" in this sense means "very good/impressive/wild". "(to be let) off the hook" can also mean a reprieve, but that's not the sense being used here.

I agree with Steve. Sometimes it’s used in spoken English in a casual environment in which you know the people you are talking to pretty well.

Hi, heidi I want to practice my language with you