You (Facebook in this case) run a hypothetical method SafeSearch('accounts.example.com') and also SafeSearch('example.com') and SafeSearch('accounts.example.com/tmp') and SafeSearch('accounts.example.com/tmp/badmojo.exe')
SafeSearch(string) is defined as, you do SHA(string) and that's your hash, you compare the start of this hash to a huge list of prefixes that Google provides, which you fetch updates for every few minutes. If there's no match, fine, done. If there's a match you ask Google OK, I saw this Prefix you sent me, what hashes should I be scared of? Google gives you a list of hashes with that Prefix. If your hash in this new list, the original URL was scary, warn users not to visit, otherwise continue what you were doing.
Sure, but this will only work for previously-known threats – for which someone else, presumably Google, has already done the request, analysis, and determination.
I doubt Facebook only wants to detect old threats, reliant on a competitor's standards & practices.
