Just to upset the applecart slightly, I find myself using abbreviations such as "u" for "you" on an increasingly frequent basis. I only use them, however, when I'm sending an SMS for the following reasons:
1. The number of characters you can send per SMS is limited (and it's not particularly big, either).
2. Using a mobile phone keypad is horribly slow.
Ironically, now that I've come to accept T9, the situation may change...
In Messenger, I tend not to use abbreviations. I don't, however, capitalise as a rule. Does this make me lazy? Possibly. Does it make what I say any less valid? No. Would it look more professional if I capitalised? Yes. Does it matter in Messenger? No.
Like everything in life, there's a balance. Abbreviations have their place, but taking them too far is, I believe, unacceptable. If you want to communicate in a public place (such as a dicussion forum), you should make an effort to be clear. Otherwise, you'll simply be ignored.