I don't think the relevant distinction here is "real life v. online." The distinction is is written communication v. oral communication. I would expect with high confidence that a Skype discussion would more resemble talking face to face, and that actual old-fashioned letter correspondence would more resemble discussion online. The big difference to me, is having the time to research and synthesize relevant evidence before responding; this tends to disrupt the flow of oral communication.
I suspect the persuasiveness in oral communication is related to our susceptibility to favorable impressions of people who mimic our tone, posture, etc.
Another important dimension is real-time communication vs slower communication. If it only takes a few seconds from saying something to hearing the interlocutor's reply, it's much easier to (say) assess inferential distances than if it takes several days. On this axis an IM chat is much like a meatspace conversation and a video message is much like a letter.
BTW, while you can't use intonation and body language in writing, you can use punctuation, formatting, and/or emoticons.
When I was younger, I thought that conversations in real life were much more likely to promote true beliefs and meaningful changes than conversations online, because people in real life were only willing/able to cite evidence they were actually confident in, while those online were able to easily search for arguments favoring their position.
While this is obviously wrong—the concept that people in real life only cite evidence they are justifiably confident in is comically false—I do think the dichotomy illustrated there is interesting. One thing I've noticed is that in general the "rigor" of discussions online is higher (in terms of citations, links to external content, etc.), but that conversations in real life seem still much more likely to actually change people's minds.
I have noticed this effect in both myself and others—what do you think is going on here, and how do you think we might circumvent it? If online discussions could be made more effective at causing people to actually change their minds, this could potentially prove extremely useful.