FAANG is used very generally to refer to these five companies. It's not just in this specific context. I'm wondering why Microsoft is excluded.
If you include MS, you have to also include IBM. FAMAING.
I see Big Four (Google, Facebook, Amazon, Microsoft) more often than FANG although that is obviously confusing on Wall St where that phrase usually refers to the large accounting firms. Apple is excluded because it is a hardware company not a software company although sometimes it is subbed in for one of the others, depending on who is using the phrase.