Agree with Tom here that the viral loop is crucial (as well as "simply" having a great product). But a big problem is getting that initial traction! And then a bigger problem is reaching a critical mass.

So how do you start that snowball?
What worked for me in the past, and is also working for startups I advise to today - is the development of automated tools that will reach out to your target audience. The market/industry is irrelevant, as long as you can identify where online your target audience is active, and then build tools around these websites/platforms.

The best case I can give you is SweetIM, where I was the second guy (Growth Hacker and Product Manager). When I joined we had a product in development and 0 users. When I left we had over 100M users and millions in revenue (check out TechCrunch story about it: http://techcrunch.com/2012/11/08/holy-emoticons-batman-perion-network-acquires-sweetpacks-for-41m/)

I've developed automated zero-cost user acquisition tools that worked and generated massive amount of users. Today I'm using the same techniques in order to generate traffic and installs for my current startup, as well as other startups I advise to.

In general I can say that you need to think outside the box! Otherwise how would your app stand out from the millions of apps out there?
Not sure what stage you're at, but Ideally you will have someone in your team that has a solid background in Product Management, Marketing, Computer Science, and a deep understanding in user-behavior / cognitive science. A person that will find the best way to reach out to potential users, he/she will know how to form habits among users, what will make them keep using the product, and what will make them invite the rest of the world!

It all depends on the amount of users you need, how targeted they need to be, where online they are active and the way you approach them. One of the problems is scalability... You may have identified some online communities in which your potential users are active, but trying to reach out to each and every one of them is a time-killer and not scalable if you do that manually!

What worked for me in the past, and is also working for the Startups I advice to today - is the development of automated tools that will reach out to your target audience. The market/industry is irrelevant, as long as you can identify where online your target audience is active, and then build tools around these websites/platforms.

In my current Startup I've used this technique to acquire 3,000 beta users for a business networking app. It didn't take long, as it was VERY targeted (converted very high! almost 10% of the people we reached out to ended up installing the app!)
This was also the case in a previous Startup - different industry, but same approach. I developed these tools for the purpose of creating a beta community. It was so powerful that we kept using it long after the beta ended. 5 years later we reached over 100M users! (around the first million came with no marketing $$ spent)

With that said, it may take some time to build these type of tools, so it all depends on the amount of users you're trying to get, as well as the cost of alternative solutions. The good news is that you can keep on using these tools for growth and user acquisition on later stages too.

I'm not sure if this is how you imagine this world to work, but at least according to the order you wrote it "raising funds" was first. In reality it should actually be one of your final steps of the stage you are at right now. It may even come after a year or two!

So you have this great app idea, and you're looking for a place to start... Don't! Don't start yet before you decide whether you have what it takes to get into a roller coaster that can ruin your life and make you miserable! Not trying to scare you but I think most people only hear about these great success stories. They have this dream of maybe, possibly, becoming the next big thing... Because they have the best idea for an app... You don't hear about the failures so often. And even if you do, you don't hear about what the founders of these failing startups had to go through.

Truth is you are most likely gonna fail. And I'm saying that without even knowing what your idea is. There are so many barriers on your way that even a great product with a great team is likely to fail. Some people would say "I'm not afraid of failing", "It's good to fail cause you learn", "Failing will make me stronger for the next startup". That's somewhat true but it doesn't mean that failing is easy. As oppose to what people sometimes say - you do not want to fail! It's very painful!!!
You have to understand what failing in a startup means. You can work your a$s for 2-3 years, have little to no salary, waste other people's money (most likely your friends and family first), lose friends, fight with your partners, your family, your spouse, devote 20 hours a day for your startup all this time, forget about the little and big things you used to enjoy in life, and only then, after debating 100 times whether you should quit or not, you finally decide that it's not gonna work and you've failed. Disappointing your family, your investors, yourself. Trust me it is painful.

Are you sure you wanna do this to yourself?

If yes, give me a call. I have the experience you need! From idea stage, to proof of concept, to running beta tests, getting millions of millions of users in ways you can't even imagine, creating features and experience that will make these millions of users completely addicted and viral, raise money in a smart way, hire the right people, find a great co-founder, succeed, fail, be persistent, and enjoy the ride!

Good luck,
Roy