As you can see, I'm addicted to the em dash (slightly longer form of "-", then en dash, I type it as "--"). I even use it when I'm writing :P
I use them in places of commans -- but I'm not sure how my head works to decide which one to use.
On IMs, I use "XD" way to much, simply because at first I had lol, then :P (which I've always loved), and XD is just the next thing. However, after setting up my client to change XD into "lmao" -- it broke my habit, probably because lmao seems "too much" for how funny it is, so I'd subconciously use :P next time. People who don't know me think I'm going into fits of laughter when I say "XD" -- even though it just means "I aknowledge that you said something that is mildly funny, but probably didn't laugh"
(Whoahh... rambling alert!)
Anyway, do you have an addiction?
(Just a note: I didn't go back and add --'s for effect, so this goes to show how much I use them :P)