Cramming these questions online does not work. I don't know what Facebook/Google et al are doing but I can spot a candidate who studied interview questions from a mile away. They may bang out a DFS by rote but that's just warmup; they should be able to talk about details, deal with changing requirements, discuss the algorithm/efficiency/design of their solution, be able to talk about underlying data structure primitives, etc.
To be fair the grandparent was talking about a specialized position in security engineering for which the generic coding interviews could be a bad match.
And I don't understand why someone who spends his time up to the eyes in crypto proofs, exploit code, and research articles should have to cram for an Algorithms 1 exam for each interview.
I've seen these questions used well (had a neat discussion with a start-up CEO in Boston once about reimplementing a toy Twitter in Scala), and really brilliantly (another start-up in Boston once asked me to describe my favorite algorithm and got a short, ad-hoc tutorial in lottery scheduling), and really, really badly (BigCos asking stuff you'd see on an undergrad-level exam from years and years ago).
Strangely enough, I'm praising the two startups that didn't offer me a job (though I'd really like to reapply to that second one), and condemning the BigCos (you know who I mean) that did offer me a job.
I'm not saying it isn't important to know, but I think there are better indicators for someone who will perform well on the job