The use of recruiters by a startup will shift dramatically over the lifetime of the company. As a small startup (e.g. 3-10 people), using a recruiter is usually not as useful as direct founder or employee networking, using LinkedIn and other tools. In contrast, when I was at Twitter, the company grew from ~90 to ~1500 people over a 2.5 year period. As your company scales into the hundreds and thousands of people, you will want to bring specialized recruiters, sourcers, university programs managers etc. in-house and potentially use retained external recruiters for executive hires.