I’ve had this happen thrice recently, I was having an enjoyable debate (which of course means I thought I had the upper hand) when suddenly people in general, and especially the people I was debating just stopped posting. I’m fairly sure I wasn’t being a dick (latest example). I’m guessing threads die on their own a lot. Does this mean people aren’t interested in the topic anymore, or that there’s something wrong with my behaviour, or what?
ETA: I mean threads in Great debates
Generally, it means that people have lost interest. Nothin’ personal. In some forums (like GQ) it can mean that someone gave an answer that was so complete that there’s nothing more to say.
If your behavior was dubious, someone would have either REPORTED it (li’l red triangle with ! in upper right corner of each post) or said something. Our folks aren’t shy.
Bottom line: don’t be self-conscious if you happen to be the last poster in a thread.
You are calling a thread “abandoned” when the last post was only about thirteen hours previous? Even your own last post was only about fourteen hours old.
I’d suggest a bit of perspective. There are only a handful of posters who hang out here for 20 hours a day, or so.
OTOH, having read the thread, I would guess that a few of your adversaries might, indeed, decide to simply drop out in the belief that you were each talking past each other and that there was no way to resolve the issue. There are a number of posters who prefer to simply walk away from a thread rather than getting into an extended exchange of “Nuh, uh”/“Uh, huh.” Your perception is that you were “winning,” but my perception is that you had simply reached a point at which the discussion hinged upon a recognition of terms that you and your opponents defined in a sufficiently different manner as to make your relative claims mutually incomprehensible.
It happens.
I’d say ‘This’, but I also wanted to say that sometimes a thread will get so long and arduous that trying to respond to a lot of the points becomes very time consuming. There have been many religious threads that I’ve participated in where debate inflation takes place and each post becomes longer and longer. In a recent thread, I had to split up one post into three posts because there was too much text for one post. Eventually you just get tired of responding or you don’t have time.
Is it possible they ran off to Facebook or Twitter or someplace else? A blogger on nytimes.com said ivillage was hurting in part because of departures to Facebook.
I noticed when I first started lurking here in Dec '08 that when I logged on at about 3pm Pacific Time and clicked ‘new posts’ that 18 pages were usually displayed. Now when I log on at about 3pm Pacific Time there are usually less than 18 pages. It is as if about between 5 and 10 percent of the posters have gone off somewhere.
Not true. The posting rate at this board has been remarkably constant at between 3000-4000 posts per day for years. Here are the stats for this year from Big Boards. The posting rate in recent previous years has been very similar.
(Oddly, the number of new members joining per day went up considerably at the end of last year (perhaps due to opening the board to Google search), but that did not result in a greater posting rate overall.)
But it did result in an approximately 3,000,000% increase in zombie threads. 
**What does it mean when a thread gets abandoned? **
In my experience, it means I started it.
This has happened to me many times.