The alleged harm caused by Meta may have a lot of merit, but the portfolio argument seems insane to me too. This line of reasoning suggests that companies have a fiduciary responsibility to avoid competing too hard against competitors because they share common owners. Anticompetitive effects have already been a concern due to common ownership by index funds [1] and would be exacerbated if this portfolio argument becomes a legal standard.

[1]: https://corpgov.law.harvard.edu/2019/03/11/the-strategies-of...