In the Southern U. S., where I live, it is not uncommon to hear "who" used in a situation where the word used should be "whom." This could be due to a variety of things like laziness, a bad habit that was never broken, or a person's genuinely not knowing the difference. I have even had some people tell me they do not prefer to say "whom" when it should be used because it makes them sound as though they are being arrogant compared to everyone else's use of "who." These issues might only be aspects of Southern culture, but I feel the need to tell you that it is better to learn to use them both correctly in their proper contexts. Sometimes native practice of English is not the best indication of what is grammatically correct (especially in the South).

No. It's not true.

That's mostly true, with "whom" being reserved for a formal register. It is especially the case when "who" is used as a question word or at the beginning of a sentence, thus "Who [not "whom"] did you see?" However, it is unusual -- and perhaps you could consider it wrong -- to say "who" after a preposition, such as in "To whom did you give the book". But even then, the more colloquial way to say this is to move the preposition to the end and revert to "who": "Who did you give the book to?".

This is true, almost everyone just says who even if it's not correct.

I use whom in sentences and I'm from the US. It depends on whether the person speaking is using "lazy" English or not, but many people do use it. It just tends to sound more formal.