[secorp]

Easiest ways that we found are to buy AdWords (or equivalent) and try out a few campaigns and to buy users from an existing source. Be mindful of the quality of these users because they may not match the exact demographic that you are looking for unless you are very particular (willing to pay $$).

We also found that it is both more difficult and more valuable to organically grow a user base that finds good utility in your service. These users typically need more word-of-mouth (including via respected blogs) or viral messaging. A good way to get people in the door is to offer a free basic service coupled with a clear path to upgrade later to a premium service when you've ironed out your business model and service offerings.

Last, we've found that a good way to keep people interested in the service itself is to keep the technology open source. This builds a very involved community, ensures that if the service is valuable that it can continue, and provides excellent references later on when you will be asked if can support large numbers of users.