I have some online friends who I talk to whilst I multi-task, they don't seem to mind as they are usually multi-tasking themselves. I usually find it is people who are insecure who seem to need you to answer them the second they type in a reply.
Personally, as I have other things to get done (especially since I started studying for my degree) I don't have hours and hours just to sit and do nothing but chat so therefore I find it easier to talk to people who don't need constant attention, who are busy themselves or who don't think that, just because I take a few minutes to answer them, that I don't like them or care.
If they want completely involved chats then they need to wait until I have spare time on my hands.
I recently lost several online contacts because I wanted to study when they wanted to chat...but the contacts I do have left are much more compatible with me in their communication demands and style, so it was probably best that I did part company (so to speak) with the others. It sounds harsh, and I am not as harsh as I sound, but the way I see it is that this may be my last chance to complete my degree and I have the rest of my life left to make friends...so do I want to jeopardise my studies simply to make chit chat when I should be working? No. I was a little sad over their loss for a while but you get over it.
Anyway, my point is that, just because you don't talk to someone constantly, or answer them straight away, it does not mean that you don't care.