Although this may be impossible to answer, I would say that most of it is related to creativity. Although the question is about literature, I think it is more easily explained through music. Many, many people have written good compositions; only the truly great have written music that is truly memorable. (This difference forms part of "Amadeus" although it may not be based in face.) There is something that inspires, perhaps controls, individuals when they create. I often wonder how great composers like Beethoven or Verdi can not only create great melodies, but then orchestrate them and sustain the effort over, in Verdi's case, 3 hours of presentation. Why can one competent individual do this, and another equally competent individual fail? How could Puccini create an opera as famous as "La Boheme" without correcting a single note? Where does this come from?
It may help explain some of the poets' praises of the muse for guidance in their writing, or for actually doing the writing. Milton tells the Muse to sing in "Paradise Lost." I always though this was a kind of humility, perhaps false. Maybe it was not.
I'm sure the social process is somehow related to literature, because you can only write about what you know. But real creative genius seems to require something else .
For more examples of the role of the muse in writing, you can see http://en.wikipedia.org/wiki/Muse
All literature, as all creative endeavors, are individual acts of creativity. Whatever environment an author finds him or herself in, may certainly influence the acts of creation; it's doubtful Hemingway would have written about life on the Mississippi River in the 1830's while driving an ambulance through World War I France. The elements within those environments, including other individuals, may serve as inspiration and may be described within a creative work. An author also may reference other creative works within their own, but those are references to other individuals' works.
Writing of a piece of literature is definitely an individual act. The degree of creativity or originality can and does differ from author to author and from work to work. But it still remains the work of an individual, even if the individual is substantially influenced by social environment.
However, reading of a piece of literature, as well as subsequent reaction to reading like discussing it with other is definitely a social process. This process can be informal - like an author passing on his writing to a friend for reading - or very formal like publication and sale of a book.
Literature also becomes a part of social process when it influences the thinking of people and their culture.