It has long been my belief that you could not convince anyone online of anything. That's extreme, the sort of rule that admits a lot of little trivial exceptions, but in general it holds. The rise of the popular use of online communication and of the political blogosphere pretends to threaten my theory, and frankly, I don't know its real status but from what I see things are unchanged and I suspect they will remain so, for it is not a feature of just the net.
A great philosopher once made the point that he could not, in fact, really enlighten his readers, for the readers would not understand anything he said except that which they had already more or less formed in themselves. If he spoke an idea that made a reader think, "aha! yes!", that was a matter of him having expressed something such that the reader recognized his own belief.
But this is not to say that people don't change as a result of their reading and conversation, not at all, it's to put the emphasis on how and why they change and who does the changing, people change themselves.
"People never change" is a popular cliche, and yet, people do nothing but change, the actual truism is "people never stay the same", and what is really meant by "people never change" is, "I couldn't change her" and similar.
If we are open to changing ourselves then the information we get from others, their facts and opinions, their arguments and refusals all inform that process of change. You may owe someone thanks, but not for changing you, rather for helping you change yourself. If it seems a subtle difference, think again, there is a world of difference between doing something for someone and helping them do it for themselves.
I believe if we all knew it was really the latter that we were doing online, it would change our approach because building a house and expecting someone to move into it is different from helping them build their own house.