There's no sectors within Facebook or Twitter that could survive being broken up (adding Instagram and Whatsapp to Facebook is just a shortcut to a feature bump, not a new market segment. As with FriendFeed and LiveRail). Not allowing them to acquire companies with competitive tech might increase competition (although instagram had no revenue when Facebook acquired it), but it's not going to stop them from adding those features into their platform even if acquiring the companies responsible is decided to be anticompetitive horizontal integration. If Facebook were prevented from sharing user data as per their current advertising model, they would no longer be financially viable (it's 89% of their income).
Alphabet and Amazon could of course be broken down, but it'd be about as effective as with Microsoft. In the end Microsoft's O/S monopoly failed to prevent the successful rise of IE competitors anyway.
Vertical integration does threaten to Balkanise services, but the regulatory policies to prevent that would have started with Net Neutrality.
To the extent that cloud infrastructure is vertically integrated there is however competition (Amazon Web Services, Google Cloud Platform, IBM Cloud, Microsoft Azure, Oracle and Alibaba). Despite the fact that in most of those cases the companies appear to be, "offering a marketplace for commerce and participating in that marketplace". Unless you restrict the definition of "commerce" to retail sales (and that would still have a huge impact, for instance it'd mean that Valve could no longer produce software/games because they operate Steam).