All large-scale, multi-user communities and online social networks that rely on users to contribute content or build services share one property: most users don't participate very much. Often, they simply lurk in the background. In contrast, a tiny minority of users usually accounts for a disproportionately large amount of the content and other system activity. This phenomenon of participation inequality was first studied in depth by Will Hill in the early '90s, when he worked down the hall from me at Bell Communications Research.
When you plot the amount of activity for each user, the result is a Zipf curve, which shows as a straight line in a log-log diagram.
This is an image
- nngroup.com
User participation often more or less follows a 90–9–1 rule: - 90% of users are lurkers (i.e., read or observe, but don't contribute). - 9% of users contribute from time to time, but other priorities dominate their time. - 1% of users participate a lot and account for most contributions: it can seem as if they don't have lives because they often post just minutes after whatever event they're commenting on occurs.
# User-generated content
There are about 1.1 billion Internet users, yet only 55 million users (5%) have weblogs according to Technorati. Worse, there are only 1.6 million postings per day; because some people post multiple times per day, only 0.1% of users post daily.
Blogs have even worse participation inequality than is evident in the 90–9–1 rule that characterizes most online communities. With blogs, the rule is more like 95–5–0.1.
Inequalities are also found on Wikipedia, where more than 99% of users are lurkers. According to Wikipedia's "about" page, it has only 68,000 active contributors, which is 0.2% of the 32 million unique visitors it has in the U.S. alone - nngroup.com 
Wikipedia's most active 1,000 people — 0.003% of its users — contribute about two-thirds of the site's edits. Wikipedia is thus even more skewed than blogs, with a 99.8–0.2–0.003 rule.
Participation inequality exists in many places on the web. A quick glance at Amazon.com, for example, showed that the site had sold thousands of copies of a book that had only 12 reviews, meaning that less than 1% of customers contribute reviews.
Furthermore, at the time I wrote this, 167,113 of Amazons book reviews were contributed by just a few "top-100" reviewers; the most prolific reviewer had written 12,423 reviews. How anybody can write that many reviews — let alone read that many books — is beyond me, but it's a classic example of participation inequality.
# See also - 90–9–1 rule * Participation inequality