How do you determine which projects will be the most profitable?

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.

Good luck.

Just pick and try one. You obviously can not determine which is better because as you said, you have no experience. And often, even the most experienced people don't always predict correctly.

Being an entrepreneur is about trying things, innovating, and putting lots of time and effort into things.

Pick something you are most passionate about within your choices, and just start rocking that thing ;)

If anything that one person could do in 3 months alone could make any vague amount of money, everybody would do it, it really how surprisingly little software one person can create in three months. I would exclude making money from the task and focus more on investigating the processes of marketing and sales, the areas developers tend to be weakest.

In fact, I'd suggest not writing any code at all, try marketing someone's open source project for example.

IMHO you should think about the niches that you'll target. By targeting a specific niche, you can lower the customer acquisition costs (we all know how expensive is to market your app).

According to "Start Small, Stay Small: A Developer's Guide to Launching a Startup" the factors that determine your success are 1. Market, 2. Marketing, 3. Aesthetics, 4. Functionality (in that specific order). We as developers tend to think that Functionality is the most important factor, when in reality the most important factor is finding people who will buy your product -that is, finding a market.

So I would go over your list of ideas, and think how difficult and expensive it would be to target each niche. Again, this is just my humble opinion.

EDIT: And here is a quote from "The Business of Software" -"software is a hit-driven business". So if you are going solo, I believe luck will have a lot to do with your success.