My textbook defines "mass" communication as communication to an audience homogenous only in one practical aspect: that of receiving the communication. The author separates "group" communication, to a set of people with many aligned interests and identities, and "public" communication to a set of people with aligned interests. A "mass" of people, he suggests, cannot be identified as having any definite characteristics.
That's an interesting definition, and very useful for certain types of communication. (Public relations, for instance.) As a general framework for the body of communication theory, though, I'm not sure it really fits with reality. It seems to me that a better-fitting approach would be to define "mass" communication as "mediated" communication.
One of the problems with current definitions of mass communication is the rise of feedback and other loops in the process. The Internet allows for flows of information that do not fit into most models. Theorists seem desperate to disparage any theory that takes into account specific technologies, but I think the technology is the essence of the communication.
Understanding the technology of communication helps us predict how information will flow, what limits it will have, and what areas it could expand to. Without technology, there is no communication. Studying communication without understanding the technology of it seems like studying the life cycle of fish without understanding the water in which they live.
So I propose a new name for the field formerly known as mass communication. Mediated communication. Telephones, mail, internet, whatever emerging media you want to consider. I think all mediated communication shares more theoretical underpinnings than difference. Each specific type of mediation allows for varying degrees of feedback, storage, reproduction, multiplication, repudiation, and so forth. But it is those factors which comprise the field we study.