Do you have more examples of brute force approaches that have worked? I think it's often overlooked, and necessary even if you have narrowed the target audience as you have suggested.
1. Monetary - Yelp paid reviewers a couple bucks a review. At Viximo we paid out guarantees. Glam paid out guarantees on minimum CPM's to early publishers.
2. Social Capital - From stories I've heard about LinkedIN, Reid Hoffman and early employees got friends to use the network and invite others.
3. Early Access/Coolness - You see this with beta invites, restricting early access, etc.
Sometimes you gotta do what it takes.