Bigger companies like Facebook don't need to let engineers know they exist, a huge number of engineers have already applied to them at some point. What they've realized is the way to hire more is to find the good ones that are sitting in the resume review stage of the hiring process, not getting looked at because they don't have stand out credentials.
One thing that's struck out to me in a surprising way about the average startup hiring process compared to larger companies is speed. We've found the larger companies are actually faster to move on the first step of booking a call to speak with our candidates. This completely reverses when it gets to bringing someone onsite and making offers though. Bigger companies take longer to do both and this is where startups have a hiring advantage.