I've noticed that I tend to avoid the period/full stop when writing instant messages like that. Like, exclamation marks and question marks go just fine, but a period will make it sound very weird in my head when it's in an IM conversation, for some reason. I will even start a new sentence on a new line without a period after the last sentence if I need to write multiple sentences in one message (which doesn't happen that often though).

I don't know why.