If i build, will they come?

If you have money, you can pay for advertising. If you don't, you will need to spend lots of time finding creative ways to market your app/service.

We fortunately live in a great time where this is very possible. But I don't think anyone here can give you a blueprint to your marketing success. It's different for every idea.

Twitter is a good start. I got 200 beta testers in a week by following people that I thought would be interested in my app. The trick is to not spam people to death.

Link your app in your profile. People like private betas because they feel like they are part of an exclusive group that gets to see your app before anyone else (and they are).

You will also have a group that is genuinely interested and can give you honest feedback. This is a key to success: build the foundation with some interesting features and have your customer base give you the rest through feedback/ideas.

It's not an easy road to follow.

First figure out a strategy to get people to use something, and then build out the product around that. For instance, I know Facebook virals pretty well, and it is through that lens that I look at new ideas - is this something that they'd post to their wall, share, like, comment on, etc. If not, how can the idea be shifted around to work? In my experience getting users is a lot more important than building a good product. I know lots of really bad products that have lots of users because they nailed the UA part. There is even one app I know that is basically nonfunctional that has over 1MM users.

Outside of FB I can only speculate, but I'd imagine that you have to think about how you could get users. If it is through ads, there is nothing stopping you from running ads before you've built the product, to see what type of ads or products people are interested in. You could find out how to get people to leave their email for invites, or what would get people to post to blogs or tweet, etc.

If you got users already, then congrats! That means you have data, and with data you can make decisions on how to improve the app. The framework I use to evaluate what to do is based on three factors, UA/virals, retention, and monetization. Let's set aside monetization. I'd look at your metrics and see if where you are weak. If you don't have metrics, then that's the first place to start.

For UA/Virals, you want to know what your conversion funnel looks like, for example how many people that land on your app become users, what channels do you get users from, and from there a/b test to make those better. You can also try to have incentivized virals, for instance dropbox gave users more space if they referred a friend. In my world it is inviting the user to get some sort of benefit or unlocking some part of the app. Once you hit upon a strategy that seems like it work, you have to a/b test like crazy because even small changes can have a big impact. If your app monetizes really well, then paid UA can be workable, basically it becomes an arbitrage problem where you try to acquire paying users for a lower cost than they will spend in the app.

For retention, you want to know what the second day retention for your app is. If I have over 10% second day retention I find that workable, above 20% is really great. The retention curve seems to be that 14th day retention is about half of second day retention, so if you get 100 users on day 0 and have a 10% 2nd day retention then you can expect to keep 5 users from that cohort 14 days later.

To boost retention you could improve your app, but that's usually not the low hanging fruit so to speak. Do you do anything to try an reengage your users, such as sending them welcome emails, or other bacon-y emails. If it is a FB app, do you send app-to-user notifications or post stories on their wall? Basically, you need to find a way to reach out to them and get them to come back to the app. It depends on your comfort level, but I don't mind pissing off 80% of my day 0 users if I can get 20% of them to come back, because without being aggressive hardly anyone will come back anyway.

You can try to force a viral marketing campaign, but if you have an inferior product eventually people will see through whatever marketing B.S. you've built.