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.
But for Facebook I don’t see how it can be broken down. Instagram, Whatsapp, and Messenger, Facebook’s three big children, are not really separate lines of business from Facebook itself. This isn’t due to defects in anti-trust laws or their enforcements. Capitalism itself just isn’t made for the wide-reaching and feedback-loop-y industry that is social media.