Being able to set yourself apart from the rest of the category is critical to your success. As a bootstrapped startup, niche targets will likely be an ideal choice since cash and resources are limited.

In addition to Tom's response. Connecting with influential blogs and those that have a following will help significantly in getting user traction based on their recommendations.

Creating videos and tutorials to solve problems or poitn out pain points that you product can address may be a low cost marketing effort.

Don't forget to test and analyze your signup data in understanding what is the trigger for your users to signup and look for ways to exploit it.

Depending on how robust your product is, you can look to establishing strategic partnerships to leverage other people's networks and or brands.

Hope this helps.