I've often found that specific threads tend to boil down to a point where the argument is going back and forth between two sole debaters (with very few interruptions from others). Inevitably, the debate gets to the point where both sides' assumptions (or axioms) have been exposed, and neither side is willing to agree with the other. To the debaters, this can be intensely frustrating. As such, anyone who has spent a significant amount of time debating others online can usually see that they're probably never going to convince their opponents. So who wins the argument?
The debaters can't decide who wins - they obviously think their argument is better than their opponents (or, at the very least, are unwilling to admit it) and so everyone thinks that they "won." But the debaters themselves don't "win" an argument, it's the people witnessing the debate that are the real winners. They decide which arguments are persuasive and which are not.
This is what the First Amendment of the US Constitution is based on, and it is a fundamental part of our democracy. In a vigorous marketplace of ideas, the majority of voters will discern the truth and vote accordingly.
Unfortunately, there never seems to be any sort of closure when debating online, because the audience is primarily comprised of lurkers, most of whom don't say anything (plus, there are no votes), and so it seems like nothing is accomplished. However, I assure you that is not the case. Perhaps not for all lurkers, but for a lot of them, they are reading the posts with a critical eye and coming out of the debate convinced one way or the other. They are the "voters" in an online debate. They are the ones who determine who won the debate. In a scenario where only 10-15 people are reading a given thread, this might not seem like much (and it's not), but if enough of these threads occur, then you really can see results...
I'm reminded of Benjamin Franklin's essay "An apology for printers," in which Franklin defended those who printed allegedly offensive opinion pieces. His thought was that very little would be printed if publishers only produced things that were not offensive to anybody.
Printers are educated in the Belief, that when Men differ in Opinion, both sides ought equally to have the Advantage of being heard by the Public; and that when Truth and Error have fair Play, the former is always an overmatch for the latter.