The internet is an amazing thing. It gives us instant and direct access to each other, whether we want it or not. Communications has become more and more an instant gratification event. My choice of which form to use varies based on my activity level, my mood, the topic, and, of course, who is on the other end of the line.
In general, I like being "offline". That is to say, I don't like being interrupted at random times, partly because I have a weird sleep schedule, and partly because I get rather focused/involved and don't like the distraction. My friends and family email me rather than call most of the time. This works out really nice for me. I can reply at a time of my own choosing.
Depending on the methods you use to communicate online, this sense of being offline can be affected.
Perhaps the most archaic of all methods. You compose a message and drop it into a black hole. You don't know if it will arrive at its destination unless the recipient responds. This is my preferred method of communicating with "the masses". I don't have to reply immediately, or at all. I can ignore it. I can pretend I am not at home, or at the computer. If I do reply, I can carefully construct my response.
One of the best forms of communication because of the persistence of information. Anyone can post and reply. The flow of each conversation is maintained for future referral. You can post when you want to, and unless the forum has some kind of "who has logged in within the last 24 hours" kind of feature, you can be invisible.
ICQ, AIM, MSN, etc. This invention put us all just a click away from each other. However, for me, it still falls firmly in the less interactive side of things. Even when in an active conversation with someone, I do not hesitate to walk away from the computer, or simply stop replying. You can also choose your own status: online, away, do not disturb, etc. You decide if and how people see you, and if you are willing to be interrupted.
This has been around for a long time (iParty, TeamSpeak, and built into various games), but didn't really become visible to the mass consumer until the advent of Skype. Just like with instant messaging, you click a button and you are connected to your buddy. Except now, you simply talk. It's better quality than a real phone call, and it's free.
This one clearly crosses the line for me. There is an immediacy to it, just like a real phone call. You cannot walk away in the middle of a conversation without being impolite. The huge advantage Skype has over a real telephone is that you decide who can call you.
What you give up in "being offline", you gain in familiarity. You can have a ten-minute conversation that would take an hour using text. You can teach complex games in minutes. You have tone of voice to convey emotion. Have you ever caught yourself spelling out LOL or ROFL over the phone?
I'm sure some day, video chat will be the standard. I know some people do it already, but it's not common. Of course, that is even more invasive. You are not only transmitting tone of voice, but also body language. You can no longer sit and chat in your underwear (unless you want to *wink*).