It's possible to build an audience for your product long before you launch it.
Invest in building out a blog that is valuable independent of your product, yet also designed to attract your product's target audience.
- 37signals - launched the Signal vs Noise blog years before the founders launched Basecamp
- SEOmoz - started as a guide blog on SEO and was used to upsell 10k members on the Pro membership plan
- Mint.com - built out the MintLife personal finance blog targeting young professionals which they then shoehorned into Mint.com when it launched