I don't think I used to make myself understand perfectly in the past, though I learnt how to express myself better with the time. I have penpals I exchange 10.000-word emails with and we express ourselves in a convoluted way, mostly when talking about feelings, ideas, ideals and topics of the sort. I do have a problem with sarcasm. I tend to use it a lot in verbal conversations, but I try to avoid it when writing. Unless I'm sending a message to people who know me well and get what I mean.
IM is more or less the same. I'm not the kind that reduces words or sentences. My messages are all little paragraphs (or not so little).
Though, now that I think of it, I might not be the best person to answer this, because I think I express myself better in written instances than in spoken ones. I'm just a listening flower pot and I just say yes/no/mmh/probably or I argue. I can't convey my ideas into words to say, but I do in words to write.
(English is not my mother tongue, so I apologize for any mistakes I might have committed)