Yes phrasal verbs are very commonly used in English and you really need to use them when you speak, and understand them when others use them .I know they are difficult for users to learn.The only thing I can suggest is to practise a lot, listening,speaking and reading and then you will improve.Try to make it fun- watching movies, talks, listening to songs. When you hear a phrasal verb try to "collect" it like something you love.This will make it easier. Make it like a game , not something hard you have to do. The more you enjoy it the easier it will become.

Native speakers use them as normal verbs, I think. Not as slang or colloquialisms. So, I guess it is necessary to learn them.

Studying lists of them may help, but I suggest writing down multiple examples and learning them instead of the verb lists.

It's one of the most difficult parts. I've made the same question to some English native speakers, and all of them have told me that it would be better not to try learning large lists. It's something that it comes naturally with practicing speaking, listening, writing and reading skills.

I understand your question and it is the same for me !!! I guess learning phrasal verbs when you're a native speaker is like when you learn the other words.It comes naturally and it takes time.You have to hear them over and over and use them as possible as you can. 

<em>Is it necessary to learn phrasal verbs?</em>

To sound like a natural speaker who is comfortable with English, then definitely yes.

Remember that a phrasal verb's meaning depends on the context, ie. the full sentence. It's more than just learning two words; you need a proper structure so the exact meaning becomes clear. :)