> Compare this to Hieroglyphics, where the glyph representive of a loaf of bread could mean either an actual loaf of bread or the abstract phoneme /t/. Specifically, a logogram represents an idea regardless of its pronunciation. This is not what we see in "text-speak"
"u" does not have the pronunciation /ju/, it has the name /ju/.
And we specifically don't see that egyptian glyphs "represent an idea regardless of its pronunciation". Look at the story of the decipherment:
> Champollion focussed on a cartouche containing just four hieroglyphs: the first two symbols were unknown, but the repeated pair at the end signified 's-s'. This meant that the cartouche represented ('?-?-s-s').
> Champollion wondered if the first hieroglyph in the cartouche, the disc, might represent the sun, and then he assumed its sound value to be that of the Coptic word for sun, 'ra'. This gave him the sequence ('ra-?-s-s'). Only one pharaonic name seemed to fit. Allowing for the omission of vowels and the unknown letter, surely this was Rameses.
That's the glyph "sun" being used because of the phonetic value of the word "sun" coinciding with part of a name. The idea goes unused (at least, the glyph is not marked with the logograph mark). That's precisely what we see in text-speak.
(Side note: while the vowel of Coptic "sun" might be /a/, this is a rare case where the ancient Egyptian vowel is known, and it is /i/. Fortunately, the general omission of vowels makes this mistake irrelevant.)
> "2" and "two" and "to" and "too" all have exactly the same pronunciation /tu/
This is not correct. "to" is a clitic; its pronunciation differs from the others, which means in particular that "to" and "2" have different pronunciations. That doesn't stop "2" from substituting for "to" on occasion.
When you see an isolated letter "u" in English, you say /ju/. When you see the word "you", you also say /ju/. This fact is what allows "u" to be shorthand or abbreviation for "you". Perhaps for you there is no distinction between a shorthand and a logogram. But, in that case, I don't see how you also couldn't argue that "you" is a logogram for the abstract idea of the second person.
In reality, we separate between logograms and alphabets by the use of symbols representing ideas or sounds. In the English example, "u" is being used as a non-traditional phonetic digram /ju/. Your example from the Rosetta Stone was a phonetic use of the symbol. There are also non-phonetic uses, where the symbol represents the sun, regardless of its phonetics. This is why it's a mixed system.
The Egyptians marked semivowels /j~i/ and /w~u/. Egyptologists use them as vowels, because they have to use something and that's as good a thing as any. We have no idea how any ancient Egyptian word was realized.
Everyone I know produces all the variations of "to" the same. Maybe there is a dialectic difference.
EDIT: I just had a thought. Let's use an English example where the shorthand is not isolated. Let's look at "r8" for "rate". This is obviously not a logographic use of "8", as the "r" is still necessary for the meaning. Instead, "8" is again being used as a non-traditional phonetic gram, the trigram /eɪt/. The use of "u" and "2" is exactly the same; their use is as phonetic grams and not logograms.
"u" does not have the pronunciation /ju/, it has the name /ju/.
And we specifically don't see that egyptian glyphs "represent an idea regardless of its pronunciation". Look at the story of the decipherment:
> Champollion focussed on a cartouche containing just four hieroglyphs: the first two symbols were unknown, but the repeated pair at the end signified 's-s'. This meant that the cartouche represented ('?-?-s-s').
> Champollion wondered if the first hieroglyph in the cartouche, the disc, might represent the sun, and then he assumed its sound value to be that of the Coptic word for sun, 'ra'. This gave him the sequence ('ra-?-s-s'). Only one pharaonic name seemed to fit. Allowing for the omission of vowels and the unknown letter, surely this was Rameses.
( http://www.bbc.co.uk/history/ancient/egyptians/decipherment_... )
That's the glyph "sun" being used because of the phonetic value of the word "sun" coinciding with part of a name. The idea goes unused (at least, the glyph is not marked with the logograph mark). That's precisely what we see in text-speak.
(Side note: while the vowel of Coptic "sun" might be /a/, this is a rare case where the ancient Egyptian vowel is known, and it is /i/. Fortunately, the general omission of vowels makes this mistake irrelevant.)
> "2" and "two" and "to" and "too" all have exactly the same pronunciation /tu/
This is not correct. "to" is a clitic; its pronunciation differs from the others, which means in particular that "to" and "2" have different pronunciations. That doesn't stop "2" from substituting for "to" on occasion.