Earlier quoted context omitted.
Features, userbase, talent; the same reasons a developer or engineer would work with any company. Let's play a game; how many degrees separated from Facebook is HN and YC? You go first, take a guess!
I can't be certain how many degrees separated the two are, but I have established a lower bound on the answer Let d:(x,y)->n be the degrees of separation between x and y. Since d(x,y)=0 implies x=y, and we know that FB and HN are not identical, we have d(FB,HN)>0. Then, since d takes integral values we know that d(FB,HN)>=1 in any case.
Your hands aren't clean here, if you actually cared about avoiding Facebook at all costs, you wouldn't be here. The fact is, you don't (nor should you).