Can marketing efforts be considered to be a factor of production? This is a sightly unconventional question, but I want to know whether marketing efforts can be considered as a factor of production. After all, if no one knows about your product, you can't make a profit. Also, marketing efforts may, in some or most cases, show diminishing returns. It is a kind of labor.
Marketing is obviously relevant. We can observe that firms spend money on marketing. If it had no effect, they wouldn't do it. Whether it is a "factor of production" depends on your definition. 

In my book, a factor of production is any input that directly contributes to the production (creation) of the good or service. That is, it enters some production function $f$ that maps "factors of production" into "output", a quantity of the produced good. In that sense, marketing is not a factor of production, because it has no direct effect on how many goods are produced. In my eyes, marketing is supposed to influence the demanded quantity, not the produced quantity. Of course the two are related, but not directly.

Belleflamme and Peitz have a nice subsection on adverstising (6.1), where they discuss three distinct aspects of how advertising (which I use synonomously with marketing) affects demand: it can be persuasive, informative or complimentary. Such a distinction matters, because the welfare effects differ. For instance, informative ads may increase welfare as consumers can make better-informed choices, whereas persuasive ads may decrease welfare as it distorts consumers' real preferences.