we use it in writing, but in the internet a lot of people specially teenagers tend to use their dialect or slang to communicate. but it is highly recommended to use the fusha (with some slang it is ok) because it is understandable by all the Arabs.
actually in Saudi dialect we generally speak fusha but we don't follow
القواعد النحوية بمعنى لا نشكل آخر الكلمات
have you ever heard a Saudi speaking? I think you will understand since you are advanced in arabic!
(actually if you are going to learn dialects it will be hard on you because there are a LOT of dialects, for example in Najed the central region of Saudi : Riyadh, Qasseem , Hail, Kharj and different towns each have their own dialect; which mainly differ in the way they pronounce words. not mentioning other regions and other Arab countries!)
Fusha is used to read books and newspapers are written in Fusha.
If you want to be understandable by all arabs you should speak Fusha because they know it (even children since cartoons are Arabic dubbed also) , and it is more sophisticated than dialects.
do you face difficulties regarding understanding Dialects? what Dialects you're familiar with?