Paul Graham wrote about how trying to come up with a startup idea is a pretty bad way to come up with a good idea. You generate a bunch of plausible-sounding ideas (that superficially resemble products you’re familiar with) that are doomed to fail. To paraphrase Mark Twain, the difference between a plausible-sounding idea and a good idea is the difference between the lightning bug and the lightning.
Facebook wasn’t the first social network, Google wasn’t the first search engine, and the iPhone wasn’t the first smartphone. More importantly, many markets are not monopolies and can easily support multiple products. The fact that someone built something similar to what you’re thinking about is a good thing. They proved it’s an economically viable idea, and you can learn from their successes and failures. The alternative (building something no one has made into a successful business before) is much riskier.
If you build it, they won’t come. Go ahead, spend the next six months building something and put it out on the internet. No one will know it’s there unless you do something about it. For a product idea to be a good idea there has to be an economically viable way to gain new customers. Maybe that’s inbound traffic through blog posts (content marketing), maybe that’s Google Ads, or maybe that’s outbound cold calls. Regardless, without a viable channel, you don’t have a product idea - you have a fun project. Check out Traction to learn more.
With large markets comes lots of money. Thus, lots of competition, a large (required) marketing budget, and a relatively small number of viable business ideas (it’s hard to make something that everyone wants). It’s much easier to build something that’s perfect for a few thousand people. Better yet, once you have a target niche in mind it’s much easier to find ideas that will delight your customers.
People are busy and won’t care about your little app. Cut through the noise and give them an experience they’ll remark about. Seth Godin has written about this.
A good way to find out if your product has legs is to build an MVP as quickly as possible and try to sell it (this is the premise of The Lean Startup, which you should read). The longer you put that off, the more likely you are to be wasting your time. Even better, know that people will pay for it before you start to build it (get them to fund the development!).