How is it "well known"? Do you have a source with concrete data? Have you asked both people who were hired and not hired if they completed/not completed their question, over a big enough sample?

Obviously, people who can't finish a problem correlate with people who aren't good candidates in general. But that's not necessary. You can have someone who shows very good problem solving and reasoning, but just doesn't make it to the end.

Thank god. I interviewed with FB, moved through the first question in 15 minutes, and assumed that it was 3 questions with 15 minutes allotted each. When he didn't give me a third one after another 15, I assumed I had spent too much time struggling with the second and he had just given up. I got the job though, just wish I had known then. I start in a few months.

I don't think that's even true, let alone well known. Unless you are talking about phone screens.

I've interviewed at FB twice and got offers both time, and I'm pretty sure I've not gotten the final answer to at least a couple questions.

My understanding is this is true for their frontend group, not fullstack.