There are two main effects of citizen journalism on the formal, established print forms of mass communication. The first involves the increased competition for an audience. For literally thousands of years, one dominant form of mass communication was eventually usurped by newer forms, often related to the evolution of technology. The more sources of information available, the more those sources have to compete among each other.
With the exception of New York City, most American towns and cities could not economically support more than one newspaper. That monopoly on the distribution of information meant considerable power for the publisher of the paper in question. As sources of information increased -- the introduction of U.S.A. Today represented a serious challenge for local papers as a source of national and international news -- the public had increased access to information, but the older, established newspapers began to see their revenue decrease.
Today, the internet provides an infinite number of sources of information. This has created many opportunities for individuals and groups to establish themselves as sources of information, often for free. Print newspapers, completely dependent upon both advertising revenue and subscription income for survival, have had to compete with the free news sources available on the internet. Consequently, newspapers are suffering financially, and the nation's most respected newspapers are struggling to survive.
Another effect of citizen journalism is increased concern about the legitimacy of sources of information. The better newspapers and wire services (e.g., Associated Press, Reuters) are generally trusted sources of information. It is assumed, sometimes erroneously, that these papers have vetted their sources for accuracy and integrity. They are highly visible, their ownership a matter of public record, and they answer directly to advertisers and readers, who pay for the product. The same cannot necessarily be said of citizen journalists, whose backgrounds, agendas and sources may not be readily apparent or knowable.