Elizabeth's discussion of communication models sent me back to the PR textbook I used in graduate school. Grunig and Hunt identified four PR Communication Models in Managing Public Relations [James E. Grunig and Todd Hunt, Harcourt, Bace Jovanovich (1984), p. 21].

The two earliest forms of PR communication, according to the authors, were one-way publicity (such as a publicist promoting a movie), followed by the less overtly commercial communication of one-way public information (such as may be practiced by a government public affairs officer).

Grunig and Hunt held that public relations functions at a higher level when it practices two-way communication. They saw corporate PR largely functioning at the level of two-way asymmetric communication, and some regulated utilities achieving the ideal of two-way symmetric communication.
Few, if any, ever achieve completely symmetrical communication. The authors saw this ideal as the ultimate future, but their model comes from a pre-deregulation, pre-corporate downsizing period when big PR staffs served large bureaucratic organizations.
Most of 20th Century PR was largely based on (and depended on) a mass communication/broadcast model. FM radio, cable TV and the Internet all contributed to undermining the broadcast model.
Much like the 19th century railroad, 20th century communication originated from major hubs.
4 Networked Communication Models
From my reading and observation of what has happened since the 'net disrupted these tidy models, we now have at least four networed models with which to contend.

One-to-One Communication: there is no center. Each node may enter into direct dialog with any other node. E-mail between individuals is the most used (and abused) form of electronic communication today. It may be strictly one-to-one, one-to-many (cc and bcc) or one-to-too-many (Spam).

One-to-Many Communication: a Web site (or a blog) may function as a one-way public information medium, but at any time an individual visitor may choose to enter into personal one-to-one communication. For those visitors who remain anonymous, Web traffic logs serve as a feedback loop, providing information on otherwise anonymous visitor preferences and user experience
Smart Web designers strive to provide opportunities and enticements for anonymous visitors to interact through Web forms; thus revealing their identity. You don’t have to be an e-commerce site to benefit from this type of interaction. For example, in both B2B and B2C sites, Web forms provide an opportunity to enter into dialog with a visitor.

Many-to-One Communication: a group of individuals with a shared interest may enter into communication about and interaction with an organization—even influence it. Individuals may post comments (positive or negative) about a product or service in a discussion list on a manufacturer’s (or distributor’s) Web site, about a movie or video game on a fan site, on a blog, on a moderated or un-moderated discussion list (e-mail) or a USENET discussion list. An organization may attempt to monitor or (carefully) influence these discussions (perhaps resulting in positive “buzz”).

One-to-Many-to-Many Communication: According to Jim Hagel, former chief strategy officer of Twelve Entrepreneuring and before that head of strategy and e-commerce at McKinsey & Company, customers don’t want a one-to-one relationship with a vendor but one-to-many vendors, plus one-to-many of each vendor’s customers. Hagel predicts “infomediaries” or agents who will broker the exchange of information on the Web between customers and corporations. These already exist in large number for e-commerce transactions.