1) They may work well for a subset of applicants for a subset of companies. This, to some, is verification that these sorts of problems are a good way of narrowing down the applicant pool. Hence why they're used.
2) From Facebook (or any other employer)'s perspective, it's an incredibly hard job determining who will fit a given job specification. You may be great on paper, but be a total mismatch with regards to company ethos (I'm not saying you are, I'm just citing this as an example). The way (and that's assuming there is a particular way) of differentiating applicants and determining their suitability is not perfect. Different companies do it different ways, and all of this is constantly evolving. The fact that Facebook is using developers to conduct their interviews could be seen as progress to some compared to what there was x years ago in y company. It's never going to be perfect.
The fact that you've moved on from this and done a bunch of great stuff is fantastic. It's Facebook's loss. But instead of just commenting on why their system sucks, and why it didn't work for you, why not make some constructive suggestions on how it might be improved.