I think the English language is incredibly difficult and causes enormous problems compared with languages where the phoneme / grapheme correspondence is closer. I don't doubt that there are pupils leaving school who have not fully mastered spelling and grammar, but I would say that the percentage is not any higher than it has ever been.
I think the literacy strategy in schools has made / is making a huge difference and needs to be given time, resouces and support from everyone.
Yes, on the whole Gransnet is more formal and literate than Mumsnet, but it is not always the case. You still see many errors in grammar and spelling and frankly I am delighted that people feel confident enough to post and are not put off by the language police and the pedants. The threads on Mumsnet zip along with people using mobile phones, often having problems with predictive text, but creating lively and useful debate. I often think Gransnet is quite boring in comparison.
Forums, texting, email are a kind of written speech, a different register, a recent genre. Do some Gransnetters perhaps not recognise that there are different ways of writing for different purposes?