Many companies in the Valley give $10K referral bonuses, for technical, design and product roles. It's cheap for them to offer that, compared to paying 20% of first-year's salaries to external recruiting agencies.

It's less for other roles.

(Source: 10 years in the valley. Made several hires, set recruiting processes at my last company, and now I run a bootcamp for technical interview prep: http://InterviewKickstart.com)