Well, one very important difference is that on HN everyone is served the same algorithm.
And one other even more important difference is that Y Combinator does not make money from HN and so have very different incentives than Facebook.
And one other even more important difference is that Y Combinator does not make money from HN and so have very different incentives than Facebook.
Everyone being served the same algorithmic results is a genuine technical difference (I believe; do we actually know that HN doesn't customise the front page for logged-in users?). I think it's quite a fine distinction, though -- both sites are trying to hold the user's attention, one just omits to use an available level of customisation. I have no moral objection to either approach.