You use slang in casual conversation, especially with friends.
Everybody uses slang.
Everyone uses slang. Name any famous person -- yes, they use slang, too.
Yes, in an informal letter it is OK to use slang.
Writers use slang because real people use it.
People use slang so that other people won't know what they're talking about.
All of them. Everyone uses slang at some time.
when we know each other very well with the same interest informally, it is appropriate to use slang.
100%. Everyone in the world uses slang.
Books and audiocassettes teaching English, French and Spanish slang.
Almost all of the language we use today in America is slang.
It depends on the type of speech. In an informal speech, slang is fine; in a formal speech, you should avoid slang.