Recruiting users occurs each time you engage in primary research activities. As you move through a project, you should always be thinking about timelines for recruiting and how that will feed into future phases of primary research.

Actually getting user feedback helps validate that a problem is meaningful, that a solution is desirable, and whether and how much they might be willing to pay for the solution.

Facebook, Twitter and Linkedin can be great places to start your search. If you're targeting a wider population, Craigslist or Kijiji may be useful as well. You should expect to offer test subjects an incentive of around $20- $50 for their participation. If you're having trouble or need to find lots of users fast, there are a number of services that will recruit users for you. Some examples are: ethn.io, User Interviews, Userlove and Respondent.io.