This only works if there's a time-limit. Facebook's doesn't have one, so they make the problems inordinately time-consuming, which has the side-effect of filtering for not just skill, but desire to work at Facebook as opposed to the many alternatives that won't make you spend the equivalent of nearly an entire workday applying. Six months ago when I applied, I spent a day completing a puzzle--and getting their puzzle-bot to accept it... it only runs every few hours, on input which is unbounded so you're not sure how optimized your solution has to be. If your solution doesn't pass, the puzzle-bot does not tell you why. Afterwards they asked me to do another puzzle at which point I voluntarily dropped out of their interview process. Overall I would say the experience was very poor and reflected badly on Facebook in my mind.