One very important rule of writing online which has not changed over the last two decades is to never use all capital letters.
This applies in forums, text messages, or these days even in social media, whether as a timeline update or comments.
Writing online in all caps was, and still is, considered like shouting.
Try writing in all caps in a forum and you’ll be told to stop shouting or you’d even be kicked out.
When writing online, you should use all caps sparingly, and with caution.
When to Write in All Caps
If your reason for using all caps is for emphasis, then writing in all caps is not the solution. You can put emphasis on one word or a phrase, but when you use all caps in your entire text, then the question is: Where is the emphasis? That’s when you look like you’re angry and would yell those same words in person. Then and only then is it acceptable to use all uppercase letters. That is, assuming that getting angry and shouting in public is acceptable!
Consider too that text in all uppercase is significantly more difficult to read than lower and mixed case text.
It’s best to write online in sentence case or mixed case, with proper nouns capitalized as well as the first letter of the first word. That is how people are used to reading printed material.
Writing in all caps is best used for short strings of words (example, title or labels) rather than full sentences. You could choose instead to use italics or bold to set off text for emphasis.
If your reason for typing in all caps is that you find it faster and more convenient, then why not use lowercase only. This may probably irritate some people, but in the internet, all lowercase seems more widely accepted than all caps.