Also, a bunch of recruiting venues exploited by Facebook are not that accessible to smaller startups.
E.g. one of the top previous employers for Facebook employees was Google (or some other outfit within Alphabet group, like YouTube). Most likely those people would've stayed at Google.
Another hiring source was university recruiting, which involves participating at job fairs at various universities, exhaustive days of back-to-back interviews, flying candidates for on-campus interviews, and eventually covering relocation costs (and potentially visas and immigration paperwork) for someone moving from Pittsburgh, Waterloo or Romania.
Would a smaller startup have the financial oomph to run a similar recruiting pipeline?