When you are speaking, you don't think about punctuation much. You pause at the end of sentences. You pause in between items of a list or between phrases, but for a shorter time. You pause between words, so that you can tell what's being said. You raise your voice in certain patterns for excitement, puzzlement or questioning. It's instinctive for most, and you don't even think about it. That's why it's so hard for a lot of people to use punctuation properly when it comes to writing.
We have spaces, commas, fullstops, colons, semi-colons, hyphens, dashes, brackets and all sorts for written communication. When I say written, I mean formally written. Letters to your mother, essays for your tutor, resignations from your job as Head Fry Chef in Floppy Burger. In this context, you can't rely on instinctively knowing how long your pause should be, or what tone you want to convey. You have to select which punctuation mark is appropriate. It's usually pretty easy. Full stops for long pauses between sentences, commas for shorter ones in between phrases and items in a list. Question mark are for questions; exclamation marks are for exclamations.
When you write online, it's different again. You have the same rules as a written letter, except you have the instant conversation factor. It's the half-way house between a letter and a phone call. Unfortunately, it's instant like a phone, and impersonal like a letter - try raising your eyebrows with punctuation. So, punctuation becomes your sole medium for conveying tones and emotions, and with the instant receipt of your message, every dot counts.
(This is obviously only applicable to those who would use punctuation in the first place, and not the type of internet user who strips words of vowels and meaning, abuse exclamation and question marks, and generally make me want to ban them from using technology with keyboard-like facilities until they learn to follow the goshdarn rules. Grammar Nazi? Only a little.)
For example, I greet a friend on Facebook chat: "Hi!"
I use the exclamation mark to convey that I am pleased to see her online, as we haven't chatted in a very long time. She tells me she is busy, and she hopes to talk to me soon. I begin writing the "Okay" in response, but I hesitate before hitting Enter. What do I put after? A full stop is rather final. It might imply that I'm in a strop with her for being too busy. An exclamation mark is even worse. A question mark is confusing and inappropriate.
This is starting to become worse than trying to decide what to wear to work today if you're my mother. She could easily spend two hours before bed, and one hour in the morning, deciding.
So, I opt for nothing. I send the message, "Okay", with nothing at the end. Crisis averted.
In fact, the general etiquette of my friends is to either fully capitalise, punctuate and write formally, or drop the capitals and use punctuation for effect. So, the full stop instead of a blank, question mark instead of a comma, it's all a new type of grammar in itself.
Phew. Isn't it tough going sometimes?