so was Digg, and Fark before it, and Slashdot before that.
Am I wrong? History shows that these sites are basically popular for a certain amount of time, and then they lose huge market share when they inevitably do something to sour the community-at-large (like trying to monetize).
You meant it hasn't so far. That's not a dig at Facebook, or a prediction of death, but on the short timescale we're looking at (a few years really) it's much to soon to know whether they've stopped the cycle, or made it slower. And therefore you can extent that to Reddit, the fact that they've gone longer without decline doesn't mean they won't start their decline tomorrow, or the next day.
But this isn't in the context of simply 'guess the future of the website'.
The context is a hypothesis that this type of site will grow and eventually die down - and assuming that hypothesis, it's too early to say whether Reddit has changed that trend or just changed how long it takes. I'd say that in another 10 years, if it starts to die then, it would have gone long enough to say it changed the trend. But right now, too soon.
Is it growing slowly and consistently, or experiencing explosive growth? The link I was replying to shows that while Reddit and Digg started at about the same time, Reddit really only took off when Digg committed suicide.