The people who care about what you've built and are going to evangelize it for you aren't your friends and family or the people who see you on an ad platform, it's the people who's problem you're solving. If you already know who they are, and if they think you're as valuable as you think you are, then add incentives to their sharing with their friends. that's why gamification is the new big thing these days...
if you haven't found your customers, and i mean real people with names, not just the demographic or psychographic that is in your business plan. If you've found 2-3 people, then get them to use your product. then get feedback so you can make their lives even easier. as part of that feedback, give them the option of inviting others into the circle.
you can also go the easy route and get in touch with a mashable or TC writer. Even a small blurb will get you a bunch of random signups, 80% of whom will unsubscribe from your list and forget about you 5 minutes after they sign up.