One way we (Zapier) got early users in the door was by trolling forums. We would manually search the internet for people complaining about integrations that didn't exist (eg. http://webapps.stackexchange.com/questions/13007/how-can-i-a...). We would follow up directly, build the integration, and instantly have a new customer. Clearly this doesn't scale [or does it?] but it put us on the right track early when we weren't sure what people would respond to.
