I agree that it's really bad etiquette to correct someone about anything
unless you have the authority over them to do so. However, it's total B.S. that people don't have time to use proper grammar. They just are too lazy or stupid to use it.
I would say that when writing in any medium, be as proper as appropriately possible for that medium, take a minute to re-read before sending, posting, or publishing, and be sure to eliminate language that could be taken in multiple ways to prevent offending the reader as much as possible. When reading, try to understand the purpose behind the writing (perhaps the bad grammar is intentional and meant to highlight something), be forgiving of non-professional writing that contains genuine typos or mistakes, and generally allow some leeway before turning into the grammar nazi. It's a two-way street.
Now, a professional post or publication, where there is (or should be) a paid editor on staff, has no excuse for bad grammar or spelling, unless it is done with obvious intent.