It's an emerging rule of thumb that suggests that if you get a group of 100 people online then one will create content, 10 will "interact" with it (commenting or offering improvements) and the other 89 will just view it.
It's a meme that emerges strongly in statistics from YouTube, which in just 18 months has gone from zero to 60% of all online video viewing.
The numbers are revealing: each day there are 100 million downloads and 65,000 uploads - which as Antony Mayfield (at http://open.typepad.com/open) points out, is 1,538 downloads per upload - and 20m unique users per month.
That puts the "creator to consumer" ratio at just 0.5%, but it's early days yet; not everyone has discovered YouTube (and it does make downloading much easier than uploading, because any web page can host a YouTube link).
Consider, too, some statistics from that other community content generation project, Wikipedia: 50% of all Wikipedia article edits are done by 0.7% of users, and more than 70% of all articles have been written by just 1.8% of all users, according to the Church of the Customer blog (http://customerevangelists.typepad.com/blog/).
Earlier metrics garnered from community sites suggested that about 80% of content was produced by 20% of the users, but the growing number of data points is creating a clearer picture of how Web 2.0 groups need to think. For instance, a site that demands too much interaction and content generation from users will see nine out of 10 people just pass by.
Bradley Horowitz of Yahoo points out that much the same applies at Yahoo: in Yahoo Groups, the discussion lists, "1% of the user population might start a group; 10% of the user population might participate actively, and actually author content, whether starting a thread or responding to a thread-in-progress; 100% of the user population benefits from the activities of the above groups," he noted on his blog (www.elatable.com/blog/?p=5) in February.
So what's the conclusion? Only that you shouldn't expect too much online. Certainly, to echo Field of Dreams, if you build it, they will come. The trouble, as in real life, is finding the builders.