In bygone times, when the post delivered mail several times a day and people wrote letters to one another, there was written informal writing. Its existence in libraries helps us understand the everyday language of that era. Online communication returns us to a world of written informal language. As always, some of the hallmarks of informality are hard to translate to writing. There are no nonverbal cues.
Inflections are difficult.
When speaking, you can change the meaning of a sentence by changing the way you pronounce the words in it. It is almost impossible to do that, or anything like it, in writing. Habits that you might have developed in informal spoken language do not always transfer well to informal written language. Speed Online communication is immediate. You can write a letter, send it, and get a reply back in a few minutes.