Breaking Google/Facebook up geographically for antitrust violations would be ineffective, but breaking them up by line-of-business would not and is well within traditional anti-trust remedies.

The expansion of monopolies into adjacent and related markets is driven by leveraging the existing monopoly to monopolize new markets, which is already within the scope of acts addressed by traditional anti-trust (whether or not one views it as enabled by “network effects” or “feedback loops”), though regulators have been ineffective at addressing it in the internet space. This isn't a defect in traditional anti-trust law, but in anti-trust enforcement.