Given that neither Facebook nor Google can appropriately police the content of their own ad networks (to the extent that they're the two biggest purveyors of malware and scams on the internet - in this rando's opinion), there is no moral argument against blocking ads.

In fact, not blocking internet advertising is a security risk.

If they get their house in order my opinion may change, but there's currently no business reason for them to change their existing lax systems; no pressure or threat from regulation to hold them liable for what they allow on their advertising networks.