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.
The problem is that the overall system is not representative of average web users. On any given user-participation site, you almost always hear from the same 1% of users, who almost certainly differ from the 90% you never hear from.