Since they increased the message size and added the ability to use carriage returns I treat the IM system pretty much like the old email. I actually prefer that the recipient isn't online when I send an initial message! I don't mind real time IM communication with someone I'm familiar with but it can be annoying with someone new.
I tend towards a message of several sentences often separated into paragraphs. If there is enough content in the recipient's profile I try to indicate that I've read their profile and think we might be a good fit. I search for and play with single guys so that's not always possible. ;-)
I'm not a fan of the fact that if the other person deletes the conversation it disappears from my message history. I loved the mailbox icon that allowed me to see email history when I looked at a profile - if it was gray or missing I knew I'd never contacted them. Now I have to look at the profile then look at my message history. I have to make greater use of Notes on a profile since messages may disappear.
~Phoebert's Wife