I think this is a subjective question with answers that vary on the person who uses each of these app. I will go with which I think would probably be the number 1 way of making an income.
I guess,, the most popular way for anyone who wants to make an income is youtube and it is usually easier to make videos and gain viewership. Many have also tried but failed as they quit this usually half way into doing it. It's not an easy thing and there is a lot of determination to make a popular video. I have tried but haven't continue. This has the highest chances of making a living as it has billions of views per month. There is a special talent that is required to make interesting content. I have seen many who have gained popularity using youtube and then after that, they just go on and become local celebrities and have a chance to be international stars.
Instagram also has many stars. This is a more popular alternative as compared to facebook and I guess to monetize this is similar to youtube in some ways. Many sites and business can use this to easily promote their products and services. The key here is get as many followers as the business can.
Steemit - This is a site which hasn't have a lot of public exposure which is good in some way as I see that this is still an exclusive club. I guess this is my preferred choice to make money as it gives us the average joe a chance. The next best thing about steem and all it's apps and dapps and interface, etc, is all these apps gives us adequate cover. I am making decent earnings at the moment for the two weeks that I have been cashing out. The whole last month, I've been accumulating all these steem and steem power. As my steem power increases, I feel that my power increases and my superhero powers increase. There is something for everyone here on steem and steemit and musing and all.
Udemy - This is one of toughest to make money. It really requires determination. I don't think I can make it. To be an instructor, we need to teach something that is new and relevant and requires a lot of planning thinking and writing of scripts. But the residual income from this will be something of those very highly paid amounts. It is a one time work. I guess, we need to have a mentor to work on things that are udemy.
These are some of the thoughts of mind with regards on these money making sites. They do make money but the only thing is that it depends on who likes what and are good in what areas.