Communities vs Audiences
Very valuable distinctions explained by Clay Shirky at http://shirky.com/writings/community_scale.html
See our related entry on community limits, cfr. the Dunbar Number
Communities vs. Audiences
"Communities are different than audiences in fundamental human ways, not merely technological ones. You cannot simply transform an audience into a community with technology, because they assume very different relationships between the sender and receiver of messages.
Though both are held together in some way by communication, an audience is typified by a one-way relationship between sender and receiver, and by the disconnection of its members from one another -- a one-to-many pattern. In a community, by contrast, people typically send and receive messages, and the members of a community are connected to one another, not just to some central outlet -- a many-to-many pattern . The extreme positions for the two patterns might be visualized as a broadcast star where all the interaction is one-way from center to edge, vs. a ring where everyone is directly connected to everyone else without requiring a central hub.
As a result of these differences, communities have strong upper limits on size, while audiences can grow arbitrarily large. Put another way, the larger a group held together by communication grows, the more it must become like an audience -- largely disconnected and held together by communication traveling from center to edge -- because increasing the number of people in a group weakens communal connection.
The characteristics we associate with mass media are as much a product of the mass as the media. Because growth in group size alone is enough to turn a community into an audience, social software, no matter what its design, will never be able to create a group that is both large and densely interconnected.
This barrier to the growth of a single community is caused by the collision of social limits with the math of large groups: As group size grows, the number of connections required between people in the group exceeds human capacity to make or keep track of them all.
A community's members are interconnected, and a community in its extreme position is a "complete" network, where every connection that can be made is made. (Bob knows Carol, Ted, and Alice; Carol knows Bob, Ted, and Alice; and so on.) Dense interconnection is obviously the source of a community's value, but it also increases the effort that must be expended as the group grows. You can't join a community without entering into some sort of mutual relationship with at least some of its members, but because more members requires more connections, these coordination costs increase with group size.
For a new member to connect to an existing group in a complete fashion requires as many new connections as there are group members, so joining a community that has 5 members is much simpler than joining a community that has 50 members. Furthermore, this tradeoff between size and the ease of adding new members exists even if the group is not completely interconnected; maintaining any given density of connectedness becomes much harder as group size grows. As new members join, it creates either more effort or lowers the density of connectedness, or both, thus jeopardizing the interconnection that makes for community. 
As group size grows past any individual's ability to maintain connections to all members of a group, the density shrinks, and as the group grows very large (>10,000) the number of actual connections drops to less than 1% of the potential connections, even if each member of the group knows dozens of other members. Thus growth in size is enough to alter the fabric of connection that makes a community work. (Anyone who has seen a discussion group or mailing list grow quickly is familiar with this phenomenon.)
An audience, by contrast, has a very sparse set of connections, and requires no mutuality between members. Thus an audience has no coordination costs associated with growth, because each new member of an audience creates only a single one-way connection. You need to know Yahoo's address to join the Yahoo audience, but neither Yahoo nor any of its other users need to know anything about you. The disconnected quality of an audience that makes it possible for them to grow much (much) larger than a connected community can, because an audience can always exist at the minimum number of required connection (N connections for N users)."