I think that, in places where the communication is not instant as say, in texting or on some sort of one-on-one messaging system, where you have the time to properly write a post out and to check the spelling (which most web browers now do for you anyway with the use of spell check add-ons), there is no excuse for poorly written posts.
Despite this however, unless a post in incomprehensible, calling someone out of having poor spelling or grammar is being pedantic and is not really worth it, if you can understand what they are trying to say. The only time this is acceptable is if they first claim a superior grasp of the language.
And YouTube doesn't count for much. I think that we all automatically devolve the second we log-in to leave comments, or something.