I know lots of startups that use recruiters, including some YC-backed ones. Recruiting can take a lot of time, particularly if you're expanding rapidly. If you're the CTO it may not necessarily be the best use of your time to look through hundreds (and yes, you'll get hundreds) of CVs from a post on StackOverflow or HackerNews...
Of course, there are some that don't, but it's certainly not unusual for startups to use recruiters.
I've worked at three startups in the valley since 1999, and all three used recruiters. Typically they have 1-3 internal recruiters, and do try to avoid external staffing agencies, but, in tight employment conditions (right now) - hiring managers tend to working with staffing agencies to get some acceleration in hiring. One of the tasks of HR, in fact, is in the growth phase from 30 to 300 employees, is to run interference and keep costs from external staffing agencies down and use the internal recruiters as much as possible.
Well, shows how much I know. I guess the fact that most recruiters I've interacted with have been terrible might be coloring my opinion of their value. How does one go about finding a good recruiter?
I also know of a recruiting agency in Japan that specializes in startups. Some companies have the too-many-resumes problem, but many startups have the converse problem. If nobody knows about you then you'll have a hard time finding qualified people.
