I run a music website on the side that I'm now completely rebuilding with a tweaked a concept (a pivot, if you will) that will be geared towards creatives and maintain by userbase -- I've had about 13,000 signups over a long period of time. When I relaunch I'll have to go through the same thought process as you, but first let me tell you what I've seen so far.
At my day job, I've run millions of dollars in paid user acquisition and lead gen. What you have to realize is that when you're paying for users, you're normally not going to get more dedicated users (simply interested people). Organic users are usually higher quality leads than users from paid user acquisition because they came to you as interested potential customers rather than you convincing them through marketing messages and a well thought-out and optimized landing page.
Paid acquisition can work really well for companies that have a high lifetime value for a customer (LTV). That's why you see ecommerce companies, for-profit colleges, insurance companies, and other similar high-priced products spending millions in the display, social, and SEM space.
Then you see the Groupons and Living Socials of the world going for a landgrab of users quickly (paying millions to acquire users at a moderate price). But that moderate price still might not back out as the category gets less sexy. You barely see ads on Facebook for these companies anymore because the cost of acquisition is simply too high.
If you're going to do paid marketing, you have to really know your business and how you will monetize your site/app to make some decent margin at the end (selling data, premium subscription, advertising, etc.). I would only use paid marketing to validate interest on a very small scale or as a small push at the beginning to gain awareness.
If you're an app that requires a lot of users who will each monetize at a small amount, you need to have a stellar product with some other distribution mechanism (partnerships, viral coefficient greater than 1, unbelievable PR machine, etc.).