Targeted advertising is not the only or even main space in which global giants avoid taxes. So not sure why such a focus on FB and Google.
Selection bias. HN community tends towards disliking Google/FB, and will tend to show articles that target those kinds of tech giants. In all honesty, they do have monopolies and it seems that the same type of anti-trust enforcement that was used to help Apple/Google in the browser war against Microsoft just isn't there anymore. Now Google/FB really do have a monopoly on user data. I think properly regulating instead…
But you're right that HN is immensely biased against Google and Facebook. That's why you frequently see commenters like yourself make the patently ridiculous claim that they are a monopoly.