There are some valid points to keeping comments for posts that have since been deleted, and I don't necessarily disagree with them. For the sake of simplicity, however, I'm assuming that all of the comments on a typical Facebook post aren't going to be relevant if the post itself is deleted. On Hacker News, however, if the URL for this post stopped working, I think it's pretty clear that the discussion below continues to hold merit.

The ideal solution to comment threads would be to replace the usernames of people who simply delete their account with [deleted], but keep the content unless someone explicitly overwrites it. The biggest issue isn't that the content is gone (any given comment), but that nobody knows how many things are missing. It would be much better (with no loss in privacy) to at minimum just replace those comments with null text and a null owner, to at least show that 3 rows used to be here, between these existing rows.