I have a basic grasp of grammar, but I am in no way excellent with it.
I'm sure that most things I write would make an English professor cringe.
I think what most people do on the internet, which is the cause for most problems, is type out exactly how they would say something in real life.
And for most people, in real life you are much more casual with how you speak.
Some people get fussy about the correct way to say things (I dislike when people use "I/Me" incorrectly when talking about themselves and another person), but for most of the time you just let things go.
If you can understand each other in person then you're fine.
And that's how people behave online.
But of course there are problems with that. Some people get very highly strung on grammar online, mainly because they aren't talking in person with someone else. And when you're distanced from someone online you tend to lose the connection with how to be socially acceptable. It's the reason why people are richardheads on the internet.
If I were to be an arsehole to everyone I spoke to in person then I would have no friends and two black eyes.
If I'm that way online the other person can just type abuse back and that's it.
People wouldn't be accepted socially if they stopped everyone mid-sentence or after everything they say just to point out the problem with their grammar. It's not done, but online it's acceptable.
Also, when people use grammar incorrectly online it can lead to confusion. There is no way to use body language or expression of voice online, so when someone reads something they have to take it exactly how they read it. If they can't read it how the writer intended it, then it leads to problems.
Suddenly something makes no sense, or it can cause offence to another person by accident or confuse them.
Think of how Furling's grammar is online. He quite often doesn't have any grammatical structure to how he types and it's simply the words he's writing which you read. And those alone don't make sense to all.
If we were all in a room with Furling and he were speaking in the same way, then we'd get a much better grasp of what he meant by his hand actions, body language and tone of voice.
I think if someone is writing professionally or trying to, then it's perfectly fine to be a "Grammar national socialist" so long as it's constructive.
"Oh, by the way, you should phrase that like this...", "Just so you know, the correct word in that sentence is normally this instead of that..."
But the in that case you're actually being helpful and not what we actually call a "Grammar national socialist".
Those are such national socialists tend to be the people who use someone's grammar as a point of attack in an argument, or who solely go about correcting peoples grammar rudely and for no reason.
Those people aren't nice.