You'll spend either sweat or cash or both.

During your drive to initial signup, I'd consider temporary use of ad placements on sites with viable traffic or leasing vacant domains for lead-gen purposes. Split the 2 halves of your demographic, and steer people toward profile creation in such a way that they add content and bring along colleagues and friends through social media.

Depending on your specific business model, you may want to do a lot of segmentation to get those really targeted spots. Billboards by the highway probably aren't for you at this point.