An overly broad definition of citizen journalism is reporting and investigation performed by a member of a group about or pertaining to the group to which they are a member of. There are several key attributes which can be applied to narrow this definition to something
smaller, but more useful.

Topics:
The first way in which we can narrow the definition is by restricting the topic to items relevant to current events, policy, or government. While a discussion of the arts could very well fall under the category of citizen journalism, current events, policy,
and government tend to be more impactful.

Amateur:
Another way to narrow the definition is to restrict the space to authors who are not professional. It is entirely possible that someone became a professional journalist because a particular issue was important to them, and their writings would be considered citizen journalism. However, amateur journalism has the property that the author is deeply invested in the issue which they are documenting, not because they are making a profit.

This leaves us with the following definition: Citizen Journalism is reporting or investigation performed by a non-professional individual on a topic pertaining to current events, government, or policy. Importantly I do not see it vital to restrict medium. The
citizen journalist can use the medium of their choice, be it video, listicle, song, tweet, or public art, or traditional article; their works may be more effective in a medium native to their community. Nor do I restrict locality; citizen journalists can report on matters
relevant to a local or global stage.

A particular instance of citizen journalism I found compelling was twitter activity during the Boston Marathon bombing. In the days after it was a very confusing period of time with poor dissemination of information from mainstream media.

Citizen Journalists took part in two major ways:

1) Reddit Investigators
The Reddit community took full force in trying to investigate and locate the bomber. There were many amateur investigators who were launching theories about who they thought did it, working with very little information. They ended up strongly pushing that Sunil Tripathi, a missing Brown student, was one of the bombers. Mainstream media ended up picking this up and also pushing out the same narrative. It ended up being incorrect, but I think that it raises very interesting questions about how investigations should be held. The Redditors were able to construct a very compelling narrative and argument that the public took seriously. Government investigators, with more resources available as well as access to information ultimately properly identified the perps, but big questions on how investigations are run and reported on have been raised, and now demand an answer. Should Citizen journalists engage in public investigations at Mass scale? Should their efforts receive more support?

This is a clear instance of citizen journalism. A community of concerned citizens looked through and performed a mass amateur investigation. While the medium was Reddit, users were writing detailed articles trying to piece it all together.

2) Event Reporters
The Reddit, Twitter, and personal Facebook networks were instrumental in disseminating information on where police were enforcing blockades, the shooting at MIT, and the chase through Watertown.  While mainstream journalism was focused on the larger picture for a broader audience, the citizen journalists were more focused on information that was immediately pertinent to their community. Personally, I spent a lot of time tweeting/posting to Facebook information from the police scanners. Friends in the Stata center were posting pictures from the Stata center, which locked down after the Collier shooting. This social dissemination of information was not only more relevant to me, but faster and more trustworthy on agenda (my friend only wants to tell me that she is safe, not perpetuate islamophobia).

This is another clear cut case of citizen journalism; amateurs reporting on current events and government responses.

The reason that I chose this example is that it is the first time that I really deeply felt that civic organization could do much better than reputable journalism. The nimbleness, lack of ulterior motive, and legitimate care to disseminate the information was very unique. It can be hard to appreciate the strength of modern information networks and the role of citizen journalists, but tragic events like the Boston Marathon Bombings highlight their strengths as well as shortcomings. There are numerous examples which bear similarity, such as the use of twitter in Egypt during the Arab Spring. But the Marathon Bombings were compelling because it was tangible; I was a part of it.