The parent said they were too powerful and needed to be brought to heel; not that they were monopolies which needed to be broken up. I think this point has a lot of merit.
Circa 2017, liberals were arguing adamantly that Russia tampered with the presidential election by exploiting the curation algorithms that these companies employ--if we can believe that these algorithms are powerful enough that they can swing an election, why should we entrust that power to a handful of companies with tightly aligned interests (irrespective of whether or not they can secure their algorithms from outside tampering)? Why should we be content with this kind of hyper corporate oligarchy? To own the cons?
Note that "breaking the companies up" isn't the only way to bring them to heel. With respect to social networks, one interesting approach would be to decouple one's social network from the specific platform through which one accesses their social network by requiring social media companies to use open protocols that upstarts can equally implement. This would increase competition, including allowing non-ad-based platforms (with all of the inherent ethical issues associated with that business model) to compete with these companies. It would also prevent these companies from shutting down alternative interfaces that circumvent their tracking. Most importantly, the competition would weaken them to the point that they're still profitable but not a threat to our democracy.
I haven't considered this possibility well enough to say with any confidence that it's a particularly good solution, but it seems interesting and appealing from this distance.