Most entrepreneurs with online businesses focus only on online marketing for customer acquisition. However, even if your business is online, your consumers live in the real world.
In many cases, especially for the initial user base, it is possible to use off-line marketing. E.g. offering it to a employee base of a company with some incentive, doing posters and leaflets on campuses, etc.
In countries like India, where the cost of customer acquisition for e-commerce sites is estimated to be around INR 1500 [USD 30], a few companies have successfully experimented with passing on a part of the customer acquisition cost to the user. E.g. Offering an INR 500 or USD 10 voucher to just signup, etc. The likely result will be higher conversions.
These methods may not be scalable or sustainable in the long run, but they can give you a decent start.