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.
"They are not the same company" is, while true, not a guess. Come on, take a real guess! When was the last time Zuck spoke at YC? When was the last time YC worked with Facebook? How many of the folks at YC either come directly from Facebook or have invested in Facebook at one time?
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).
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.