[What follows is first draft of a section from my forthcoming book.]
*The mass media, as an autonomous social system, is a relatively new thing; and (although already in existence) was only recognized as an entity in its own right (The Media) from about the nineteen fifties by Marshal McLuhan.
The mass media are defined by communications which go from one to many persons (or from a small group to a much larger one).
In sum, a mass medium is a system of amplification for communications: such as a printed book or newspaper, a radio or TV program, an internet blog or the 'social' media such as (written in 2013) Facebook and Twitter.
Before the Mass Media, there were several mass media – and some of these reached quite a massive scale of amplification such as the lecture, the play or gladiatorial and sport spectacles such as chariot racing – which in Roman times had reached an amplification rate of one to many thousands – thanks to the technology of the amphitheatre.
Writing is potentially a system of amplification since it allows for copying, but the most famous mass medium is the printed page – credited to Gutenburg's invention of moveable type.
*But in these early times the mass media was simply a range of technologies for amplifying communications – and the communications originated from other social systems that has the usual social functions; systems such as government, the military, the legal system, the various arts, and scholarship (such as theology, philosophy and science).
Early media took their functions from the social systems they served. There was no single Mass Media, and the functions were as diverse as informing and entertaining – for example when mass media amplified government – perhaps in writing by pamphlets or through newspapers, they might provide information, or provide a conduit for propaganda – intended to shape behaviour, or perhaps provide some kind of ethical inspiration or guidance.
When a mass medium amplified science it was perhaps educating via a textbook, informing via a scientific paper, or may be popular science in a newspaper or radio broadcast. When a medium amplified the arts (e.g. by printing a novel or poem, or broadcasting a play on television) it could be proving entertainment, or an aesthetic experience.
At this point, therefore, the various mass media had no unified function – they were merely mechanisms for amplifying the communications of functional social systems – so it could be said that they served to do something along the lines of conveying information, aesthetic experience, entertainment and propaganda.
However, once the various mass media reached a certain size and began to cross-communicate, then the system of mass media communication began to communicate with each other; that is to refer to, and to react to, each other.
At this point the Mass Media could be considered a separate system. It was no longer just a mechanism for amplifying the communications from other systems, but the various media reacted to stimuli from each other – and the output from these was... more reactions. The Mass Media was a system, and the system was (potentially) autonomous.
So, a newspaper runs a story – and this story could originate from almost anywhere – discovered by the mass media's own 'reporters', from a press release, from a rumour – it does not matter; but this story is repeated in the broadcast media and across the internet and evokes reactions from all these sources – leading to stories about the story; and any or several of these stories about stories may lead to further reactions – and so on.
Thus while the old mass media were merely amplifiers; the modern Mass Media is substantially independent of the other social systems. Whereas the old mass media would inform – because it was simply telling more people what other social systems had generated; the modern Mass Media select, re-shape and just plain invent outputs which are 'designed' (intended) merely to evoke reactions from itself.
Therefore while the old mass media had not intrinsic function, because it was not a system; but merely a set of amplifiers; the modern Mass Media has no intrinsic function because it simply generates outputs to evoke reactions from itself.
But his is not, of course, purely technological: humans are necessarily involved. The constraint upon this is that people must be induced to participate cognitively in this process of reacting – the system of the modern Mass Media must therefore include human minds, as well as technologies. Somebody must read some of the newspapers and react in some way – whether by buying, or gossiping, or voting, or rioting – and thus provide feedback stimuli thereby to close the loop and re-fuel the Mass Media
The point is that it may at one time have been reasonable to summarize the mass medias functions as (say) informing and entertaining – since the mass media took information perhaps from science and amplified it; now the Mass Media generates stories which it references to science, but these stories do not have to be true – certainly the stories do not need to be true according to scientific criteria. Media science stories are simply references to science, and may variously be true or selected, distorted or invented as seems most likely to provoke Mass Media responses some of which will lead on to further Mass Media responses – of a type that engages sufficient people in such a way as to fuel further communications (buying more newspapers, generating advertising revenue or subscriptions or buying more equipment or whatever). But there is no reason why a science story should be true.
Similarly with entertainment. For traditional mass media to amplify entertainments they generally had to be enjoyable – to sell a lot of novels, people had to enjoy the novel; to get a lot of people to watch something on TV is needed to make people happy, or excited or make them laugh or something... But in the modern Mass Media, entertainment does not need to entertain; so long as it compels some kind of attention this works just as well as entertainment; and since it is difficult to entertain people en masse and for long periods, there is not much entertaining going-on.
So although there remains an element of entertainment the modern Mass Media attract attention by any and every means: by evoking disgust, horror, fear, lust, repulsion, self-satisfaction, pity for others, self-pity, hero-worship, scapegoating... and then reacting to these responses, and reacting to the reactions.
The most typical modern Mass Media event is therefore some kind of staged pseudo-'reality' show, consisting of people who evoke strong reactions, engineered into situations designed to evoke responses – which may then be displayed to elicit further responses. These shows neither entertain nor inform; but are calculated simply to attract and engage attention by whatever means, and evoke opinions and behavioural feedback which may be harvested and channelled into an iterative process which serves nothing beyond its own growth in communications.
(The above is a response to WmJas's suggestion in a comment that the Mass Media did have a function: namely to inform and entertain.)