It's not an easy question. One thing most people don't understand is that you first have to find a buyer before buying the product and put it in storage. Instead, they buy a variety of goods and hope that someone in need of something, may buy something else. For sure you can do that if you have money aside.
You may put a lot of efforts developing something, gathering some data, organising it in a proper way so someone can take advantage of this and want to buy it or buy the program. Someone? The time is the most precious thing. You can't take it back if you spend it unreasonable. Do your research, ask for an advice people who are doing something like that. I am not in the business.
The reason for which facebook is estimated at more than a hundred billion is all that information which people are sharing. People likes to brag. About the new car, new shoes for all which is positive in their life. And that is a very, very valuable marketing information. That is the reason every phone app is asking you for permission ( if asks ) to access the contacts, the messages, the location and so on. Even the OS itself is gathering all of this. Google is doing well too.
Of course, we are not talking about such a scale. But this has a potential to grow into something huge.
This was a lot of typing for me. Apologize my English!