I've used mechanical turk in getting initial users. You can set up a task, and have turkers do very specific actions on your site, for $.05. And, studies have

I've used mechanical turk in getting initial users. You can set up a task, and have turkers do very specific actions on your site, for $.05. And, studies have been done, that suggest that Turkers are really very well educated, and are probably early adopters. If you treat them with respect, and show appreciation for what they are doing, they will go the extra mile for you. I've also used Adsense to get initial users. For my keywords, they cost about $.10 a user, and about half of them bounced. But, it provided me with a demographic that I wouldn't have reached with Mechanical Turk.