You are anonymous here, and your post contains no personal information, and none of your real life friends and acquaintances will see any information you intended to keep private.

With facebook, you are not anonymous, many of your posts would contain private information, and many of them could be pushed to people you didn't want to share with.

I don't know what threat models you find compelling. But the damage HN could do to someone with the info they have from an account holder is an order or two of magnitude less than the damage facebook could do.