Here it is: "The signal is highly concentrated in the early adopters of every publishing technology."

That is to say, the people most likely to have something interesting to share are the ones busy trying to find new ways to share it. Thus, when the Internet first became available to the masses, the early adopters were people with things to say but no easy way to share their ideas with people interested in what they had to say. So they put up home pages and essays.

Then the masses came on board and businesses catered to them and we got FrontPage and GeoCities and the signal dropped to zero. We can see the same thing in every forum, in Slashdot, in Reddit, and here. Early adopters are invariably chagrined to see the laggards driving the signal to noise down, some leave in search of new media with an opportunity to enjoy the high signal of halcyon days. Eventually the older media have an Eternal September, and the combination of high signal early adopters leaving and a sudden wave of new laggards joining the signal down to nearly zero, and suddenly everyone is dissatisfied.

While not every early adopter has something interesting to say and not every laggard is taciturn, the proportion of writers to readers is higher amongst the early adopters of any communication medium than it is amongst the laggards, which creates the phenomenon you observed.

That can't be entirely right, can it? A medium's early adopters are really the most likely to have interesting things to say? How can the two possibly be correlated? I agree that there's an obvious pattern between signal:noise ratio and community size, but my gut tells me the reason is likely more than "the early adopters were more interesting."

I can't find the source now, but I recall reading an article that posed another explanation: communities start out small, and the members know the rules/mores/norms and abide them. When a newb steps out of line, the graybeard:newb ratio is small enough that it's not hard for the community's graybeards to reinforce the rules with the him. As the community grows, the definition of what's interesting to it gets more vague and distorted, like a drawing on a balloon that's continually inflated. And if it grows quickly, the graybeard:newb ratio drops. Both factors make it harder to enforce the interesting-ness norms.

Since you are asking for a possible explanation, and not a provable one:

People with lots of good things to say naturally seek out places where what they say is not drowned out by meaningless noise. This means that whenever a new publishing medium is established, they jump on it in order to say their good things before Zynga figures out how to use it to broadcast pleas for help milking virtual cows or whatever.

I think this is a version of (or at least, related to) the concept of Eternal September.

http://en.wikipedia.org/wiki/Eternal_September

Or, more generalized: "For any consumer, signal is highly concentrated in the publishers who share your adoption speed of a publishing technology"

I guess the ease and ubiquity of Facebook has lowered the barrier of entry to publishing content to the Internet. Consequently the average quality level of content decreases, despite its added value of being personalized.