I've never tried it myself, but I've heard people have had decent success using a combination of Twitter searches + live chat.<p>You can monitor people on Twitter complaining about the problem you solve, @reply them to check at your site, and then chat on oLark/SnapEngage to help convert them into users (or just do customer development).
I think SlideShare is one of the most underutilized tools out there. It's a great way to break up a lot of information (people enjoy clicking through) and they have a vibrant community. I post things up there and routinely get tens of thousands of highly relevant views.
"Meetup.com is one of those services that will do the job of driving traffic to your meetup if you create one." This is a great point, but I think it's also important to focus on creating successful meetups. This is a huge time commitment. Creating a consistent arena for your user base requires coordinating activities that they would love, and then reaping the benefits of customer loyalty. It's an excellent idea, but you need to devote a good deal of time to it (sometimes even a person).
Great article, but in your Craigslist section, you could mention how AirBnB used Craigslist to get their first houses listed <a href="http://davegooden.com/2011/05/how-airbnb-became-a-billion-dollar-company/" rel="nofollow">http://davegooden.com/2011/05/how-airbnb-became-a-billion-do...</a>
I like the way you put it - "Put simply, go to where your potential users are".<p>I would add - Think about not just users, but power users! At-least in the begining you are not looking for a regular joe, but somebody who would benefit the most by your product, and be the cheerleader moving forward.