If you could hit a button and be at their doorstep, the vast majority of reasons to chat rather than going to visit them would still apply, though, right? Going to visit someone has a major overhead, socially. A quarter hour of conversation, at the least, before you can politely get to the subject of the visit, etc. Phone conversations have some of the same costs to a lesser degree: it's quite rude not to devote your full attention to the call (or appear to do so), and there might well still be a few minutes of overhead. With IM, you can almost just start talking. It cuts through all the extra junk around actually having the interesting part of the conversation with a person, but you can still have an actual conversation (even while you both do other things as the other person types), which is important for discussing things at length.

Basically, I would say that calling and texting have very different social costs, but calling and visiting are much closer that way, and that this is why it's almost always better to text than call, even for extended conversations.