I'm a US native speaker, and I'm retired.

By "memos," I mean written communications that are older and more formal than texting. I am thinking of things like written interoffice memos--on paper or in email. They are in an everyday register, neither informal nor formal, and appropriate when used peer-to-peer, peer-to-supervisor, and supervisor-to-peer. Anything used in memos is also used in modern electronic media like texting and social media.

<em>ASAP </em>is used in memos. It is also spoken, but only occasionally, and not by me. When spoken, it is pronounced aloud as a word, "ay-sap."

<em>FYI</em>, is used only in writing, in memos. I have never heard it spoken.

<em>BTW, IMHO, IMO, BRB </em>are used only in modern online social media (texting and online forums). Others in the same category include <em>YMMV, IIRC, AFAIK</em> ("Your mileage may vary," "If I recall correctly," "As far as I know.")

<em>ETA </em>is used In writing, and in speaking. <em>ETA</em> is pronounced as an initialism, "ee-tee-ay."

I think <em>ASAP</em> and <em>ETA</em> began in the military and go back at least to the Second World War, if not farther.

“ASAP”, “FYI”, and “ETA” are heard in regular contextual speech. The others are internet slang, and would only be spoken ironically.

ASAP ETA I use only in speaking and I used it daily on a radio to talk with a controller.

ASAP FYI ETA I would only occasionally use in texting or social media apps.

I would rarely write them except on odd occasions in a note or memo pen and paper we used to use before digitalisation.

I agree with Irene and Ted (ASAP, FYI, ETA) and would add that ASAP can be either spelled out or said as “ay sap”. Personally I would spell it.

I've heard people say ASAP, FYI, ETA and BRB in both texting and speaking, although BRB is not really said by adults, mostly young adults and teens, if at all.

ASAP is usually said as a word, like "ay-sahp" and FYI, ETA, and BRB are not, people say the letters.

IMO and IMHO are just for texting and in online forums, just variations of each other really.