There is probably a better answer to this for projects taking a large effort, but I have always followed a simple formula for smaller one-man projects.
Figure out how long it would take you to get the project live. Next, figure out how much money you would have to make to make it worth your while. From there divide the number by the per user payment and you have the number of users you'll need to reach the goal. For a mobile app I think this would be the number of paid downloads, for the FB app I am not sure as it would depend on your business model. If its ad driven you'll need to assume conversion and CPC rates.
Either way you'll end up with a ballpark number of users, time estimate, and estimated revenue. For example, if the mobile app is .99$ and you want to make 10k over the summer you'll need 10.1k paid downloads. For the FB app using something like premium item purchases with a conversion rate of 5% and an average lifetime customer value of 20$ you'll need 500$ paid users, and about 16.6k users.
Now I have pulled these numbers out of my proverbial behind but you can see at the end of the experiment you should be able to decide which is more realistic and worth your time. There is however an overriding factor, and that is when you are really passionate about one project over the other. If that is the case than forget what I said and just do it. Your passion will help you gain users and make money faster.