Well, I think you've pretty much answered your own question here. Such codes have long been the norm in text messaging (and, I think, to a lesser degree in IM). Although it's not so much the case now, I think this happened because until recently most people did not have full keyboards on their phones, which makes such shortcuts very tempting.

This isn't a new thing. There are plenty of rather opaque, mostly forgotten abbreviations from the era of the telegraph. Take a look at this fascinating list of telegraph codes.

If you scroll down to "page" 9 in that article, you'll find that it is recommended that beginning telegraph operators first learn the single letter codes. And what do you think "U" was used for? That's right, "you". "R" for "are" is also listed there.

Many people (myself included) think that using such codes in ordinary correspondence is highly annoying.

Well, I think you've pretty much answered your own question here. Such codes have long been the norm in text messaging (and, I think, to a lesser degree in IM). Although it's not so much the case now, I think this happened because until recently most people did not have full keyboards on their phones, which makes such shortcuts very tempting.

This isn't a new thing. There are plenty of rather opaque, mostly forgotten abbreviations from the era of the telegraph. Take a look at this fascinating list of telegraph codes.

If you scroll down to "page" 9 in that article, you'll find that it is recommended that beginning telegraph operators first learn the single letter codes. And what do you think "U" was used for? That's right, "you". "R" for "are" is also listed there.

Many people (myself included) think that using such codes in ordinary correspondence (that is, anything other than texting and IM) is pretty annoying. But certainly in TM and IM it's much more widely accepted.