It's an interesting question. I assume human nature has more or less remained constant in its vulnerability to be mislead.
Presumably, people may have been somewhat easier to trick when the populous was less educated, more superstitious. And there were fewer information sources--basically just print and word of mouth. Control one or the other and you're half way there.
Now we have more information sources than ever, much of it at our finger tips. On the other hand, the art of misleading has become more sophisticated, and there's little to stop its almost instantaneous distribution. This momentum, in and of itself, makes it more believable.
It seems like the hacking problem: computer defenses are constantly improving, but always a step behind the hackers.
I saw a talk where George Will argues that eventually people will get tired of Facebook/Twitter. Well, they may grow weary of a tweeting president, but I'm afraid viral information is going to continue to sweep over the land, for better or worse.
