The OED says that “literally” as “figuratively” has been around for over 200 years. No one else has been confused about what it meant in context.
You yourself indicate that “literally” is not meaningless. It’s just a word that has two different and contradictory meanings. Look up “contranym.”
I hear this from time to time. (About Chinese, too!) Yet an English grammar book pushes 2000 pages. But it does not prescribe what words to use and how to make them do what you want, and style varies quite wildly, of course.
Because, as you say implicitly, grammar is not usage.
Grammar, usage, spelling, pronunciation. Related, overlapping, yet ultimately distinct. When people try to equate them, the result is a school and class system that is used as a weapon against the unprivileged.
And not just in America.
I do find it ironic that the guy whining about people being illiterates is abusing the term “illiterate”.
Wouldn’t that be “illiteralates”?
It’s okay when he uses words figuratively, I guess. 
I’m another one who has a problem with the usage of “literally” as a generic intensifier.
Yes, I recognize that sometimes people use figurative language. And usually, that’s no problem, because usually, it’s clear from context that the usage is figurative. Occasionally, though, it’s not clear from context whether a usage is figurative or not. And that’s still no problem, as long as there’s some way for a good communicator to disambiguate those cases. “Literally” is often used to serve that purpose. But it doesn’t work if the word “literally” can, itself, also be used figuratively.
If someone says “My head literally exploded”, well, of course I know that’s figurative. In that context specifically, the use of the word “literally” as an intensifier does no harm. But if someone said “I’ve written literally a thousand pages on this topic”, well, that just might be true. They might be the author of the definitive scholarly tome. Or maybe they’ve just composed a dozen posts about it on message boards. I can’t tell. And it’s very difficult for the speaker to resolve tht ambiguity.
It is exceedingly rare that I’m confused with what is meant by “literally.” That said, it finally happened a few days ago when I found a baseball clip of someone “literally hit a homerun to the moon.” I was actually expecting the uploader in being cutesy, and that a homerun hit a sign in the outfield or something with a moon on it. Something of that nature: setting up the expectation of literally as an intensifier, but subverting it. Alas, I was disappointed when that wasn’t the case. 
But that’s the only real-life example I’ve come across in many years. There was another one here posted on the Dope one of the umpteen times we’ve had this goshdarned discussion that was ambiguous. But 99% of the time, it’s clear. It’s so rare, that I remember these individual cases.
If it’s the former they’re trying to communicate, what about “I’ve written the definitive scholarly tome on this subject.” If it’s the latter, they could say, “I’ve spent a lot of time arguing about this on Facebook.”
Seems pretty easy to me. There’s no end of ways you can say something ambiguously, regardless of how carefully you fence your definition of “literally.” There’s also no end of ways to reword things to make a meaning more clear. It’s on the speaker to be aware of potential ambiguity - if you’re in a rare situation where it’s both unclear and actually important which usage you mean, find a different way to express your meaning.
My specialty on Facebook is ambiguous one-liners.
“Illiterate” can be an adjective or a noun, and encompasses the meaning I intended, particularly definition #3.
So, not literally illiterate…
Maybe they’re just describing how illiterates use the word.
No, quite literally illiterate. The word was used precisely in accordance with the definition in Merriam-Webster, as noted before. I was responding to someone accusing me of “abusing” the word, who apparently doesn’t know what it means.
So you’re justified in your exaggerated use of the word that deviates from the original definition because Webster’s Dictionary says it’s ok?
Not just Merriam-Webster, but every other dictionary I’ve ever seen. It’s not “abuse” to use a word as formally defined, nor is it “exaggerated use”. Did you even bother to look it up in the dictionary?
If “illiterate” ever meant exclusively “unable to read and write”, it has long since evolved to the closely related meanings of “not well schooled in literacy” and, by extension, to also mean “not well schooled in some other field of knowledge” – e.g. “scientifically illiterate”. Unlike the discordant use of “literally” by bad writers to mean its exact opposite, these different nuances of “illiterate” are all closely and logically related.
“Literally” isn’t used to mean its opposite. It is used in exactly its normal meaning, the only difference is that it’s an exaggeration. For example, if I decline to walk to my bedroom from the lounge to get my phone because it is “miles away”, I’m not redefining “miles away” to mean something quite close instead of far away, I’m using the words exactly as they’re normally defined but I’m exaggerating. If I say my phone is “literally miles away”, then “literally” has just become part of the exaggeration, an intensifier, but it is still being used in its normal definition just the same as “miles away” is.
I basically agree with you, but my objection is somewhat more nuanced and has more to do with the misuse of the word by incompetents, which is why – as I tried to explain earlier – it is no contradiction at all that I approve of its metaphorical use by good writers and ridicule its misuse by illiterates. Let me give this another shot.
The second meaning of “literally” is, as I believe you’re suggesting, supposed to mean “virtually”, or “nearly”. As such, good writers have used it in that sense to evoke an appropriate, expressive metaphor that works, and the reason it works is precisely because it’s usually close to the literal reality, as I said before, and gave many examples.
When bad writers try to ape this usage, they seem to have come away with the impression that “literally” is just an all-purpose generic intensifier. So they generally end up with discordant drivel where the word really does appear simply to stand in for the opposite of its usual meaning, and accomplishes nothing whatsoever except potentially confusing the reader. That’s what makes it bad writing, and thus a stylistic choice that IMHO unskilled writers should avoid.