How does one win a debate?
Who is truly the winner of the debate? The person with the last word or the person the makes his/her point the clearest to the ears of on lookers.
There's no mathematically quantifiable way to tell who won an argument. You'd have to judge by the content. My own interpretation:
When person A is still producing reasonable arguments (backed up by reliable evidence and logically acceptable), while person B has degenerated into mindless insults and is unable/unwilling to adequately reply to A's arguments, I would call it a pretty clear win by A. Which party left the debate first is immaterial; the winning party could well have left because they were sick of the other guy's idiotic nonarguments.
When A and B were both producing reasonable arguments, then B suddenly disappears, that often means B realised they couldn't make any more reasonable arguments and hence shrank away. Not an absolute though.
When both sides are mindlessly mudslinging each other, I'd say both sides lose :P
And I think there are times when there is no win or loss, e.g. when the two parties start off with very different viewpoints, and after some discussion they amiably agree to disagree, or they settle for a shared perspective.