I think it's hard to say whether texting or IM'ing is more intimate. There's definitely something more personal about getting a message directly on my phone; but it's true that the back and forth interaction is less.
(This line will probably disappear entirely as phones become more sophisticated)
Yeah, agreed. I rather think the main message of this graphic is to show how far away we're by now from each other. So the details whether texting or IM is more intimate are not crucial.