When I was younger, I used to say things like:
"It's me!"
"Whom are you?"
"It's her!"
"Whom is it?"

Simply because that's the way people talk in everyday life, and I wasn't particularly knowledgeable about grammar.

When I got into my twenties, I learned that—according to the rules of grammar—"to be" takes no object. Yet I still didn't change the way I spoke, because it seemed horribly pretentious and detached from real life.

However, now that I'm older, I feel differently about this. I'm starting to appreciate correct speech and correct grammar more, because I find it more dignified, more polite, and I just like the sound of it more.

Therefore I have found myself saying things like:
"It is I!"
"Who are you?"
"Is is her!"
"Who is it?"

As for the popular opinion that "whom" is archaic: I don't agree and, frankly, I don't care. I find "whom" completely natural to use in everyday speech. On the other hand, to me, sentences like: "My friends, all of who I love." just sound like terrible English, and totally jarring.