Why should every software company manager study digital marketing?

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I have been working as a software engineer for almost 7 years. 2 years ago, I had a new software idea that I thought people would love it. I spent over 1 year developing this software and finally I launched it a few months ago. The software is a competitor to a famous software company that has more than 50K customers. 
The application has a free version to encourage people to install it and I thought it is enough for marketing the application but the application now has been launched over 6 months ago and only 100 people have installed it. As a software engineer, we focus on building the apps with creative ideas but we don't have any idea about how to market it.
I started to ask myself how this software can gain much more visibility. How can people know about it more? Promoting on Facebook, Twitter or LinkedIn? Hiring a motion graphics agency to have a nice video for the application? Focusing on SEO? What's the most important thing should I focus on? Should I start investing on digital marketing while I don't have any idea about it? What if the agency I hired costs me a lot of money for a little revenue and a few new customers? How can I measure the success for any proposed marketing plan?
While spending my free time on social media, I have stumbled across a sponsored post for "Digital Marketing Nano Degree" program from Udacity. I thought that this post came at the best time because I think that this degree would give me the appropriate knowledge and the answer for all of my questions!

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Thanks to your feedback. 

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