I have two options when I see my friends online. Sometimes, I leave them alone because I don't have anything to say to them, I'm busy or I just don't feel like talking. Sometimes, I drop them a line. Then I wait for a response. If they respond and are not busy, we chat for a short while. If they respond and are busy, we just say 'hi' and 'bye'.
If they don't respond, I sometimes wonder why. They are not at their desk? They don't want to talk to me? Whatever it is, a response, even if it's "sorry, I'm busy, can't talk", or "sorry, I'm not in the mood to talk to you" is better than a rude silence. Or there is always a status indicator that you can select or customise to notify others that you are busy, in a meeting, not at your desk, on the phone etc.
And if you can't continue chatting, please inform the other person. Don't just abandon the conversation. That's not nice at all. It's like leaving the phone off the hook halfway through a conversation and disappearing.
I think that technology is one of the reasons why some friends are not as close to each other as before. They think they can continue the friendship online via email, chat, social networking sites like Facebook...but it's just not the same, unless both parties make an effort to keep up with it.
I think it's about time I do some housekeeping with my chat and Facebook contact lists! Or maybe just forget about chatting online. Whatever happened to the good old days of meeting up, chatting over the phone or writing letters? I used to love the feeling of excitement when I find an envelope with a familiar handwriting of a friend in my mailbox.