Another problem with judging someone's literacy from their online posts is factoring their typing ability. I write some of the most proper British or American English technical manuals, short stories and poetry. But I am not a good typist. Sometimes my fingers are almost dyslexic and my laptop's keyboard is drastically different from the one I spend hours using at work.
Professional proof readers know that the hardest writing to proof read is that which you have written with your own hand and mind. Ergo, grammar and spelling may be completely corrupted by one's keyboarding skills.
