The question of whether a certain usage is appropriate depends entirely on what the audience expects. Look around you, and if most people are using it and don't seem to disapprove of such abbreviations, feel free to use them. Conversely, if typing out full words is not much trouble to you, feel free to avoid them — especially if you suspect that even one person is going to have a slightly negative reaction to them (which is likely).
I think they are perfectly acceptable in IM and text messages for informal conversations (e.g. between friends), and possibly more formal (text messenger) conversations as well, in certain contexts. I don't use them myself, because I can type fast enough and it would actually take me more time to pause and consider which abbreviations to use, but I accept that some others are used to these abbreviations, and after years of having a negative reaction to them, can successfully parse them at normal speed these days!
For emails sent to many recipients, or messages posted online where they will be read by many readers, it is always worth making your writing clear: your little time improving text is worth less than the little extra time many readers would take to stumble over unfamiliar usage. Avoiding such abbreviations is then part of such general etiquette.