Short answer: it's not you, it's me.
Long answer: I get pretty anxious when it comes to one-on-one conversations with people, especially in settings isolated from everyone else. There's a constant pressure there to keep the ball rolling, and I'm not very good at coming up with immediate responses. Which is part of why the internet is so great--I can communicate with others by taking some time to think and formulate a response without the other person being subjected to a period of awkward silence.
...not so much with IMing, which is basically RL conversation in the form of text. I can see where it has its advantages that RL conversation doesn't, don't get me wrong; you can still get your thoughts down in a way that RL can't, and nothing can break the ice quite like passing each other links. But there's still that pressure to keep the ball rolling and come up with some form of response, even when you're not sure how to respond. And then there's stuff like this (excellently summed up by my friend the Socially Awkward Penguin) that's constantly occurring and...see what I'm getting at, kind of?
This is also why I like chatting on Livestreams best, because, in addition to having a film or game actively going on one screen to keep the discussion going, the group setting allows for that pressure of a select two people keeping the conversation alive to be removed. I know that in the past when I used to use AIM, I would try to get a chat room started as soon as possible for that very reason.
So, there you have it. Hope that made sense and sorry if I might have accidentally gave the impression I didn't want to speak with any of you (quite the contrary, actually)! Again, it's not you, it's me.