Now why do I think they deserve Sherman Antitrust investigations (and IANAL).
All of them have control over people's different dimensions of data that nobody has.
- Amazon: With acquisition of Whole Foods they control retail like nobody. Yes, Walmart is trying to make a dent. So does best buy. But they have pretty much monopoly over e-books business when it comes to publishing. Apple already had eBooks settlement.
- Google: They make or break Internet with control over gateway (Chrome), Search (gatekeeper, and promoting their own content) and monetization (Google Doubleclick/Adx is majority of share in ad-tech market for on-walled garden s, good luck competing against it). Given their control and 1B users across multiple platforms they constantly engage in anti-competitive behavior. Read about rise of header bidding and how AdX used to anti-competitive in ad-tech. Google's ad-tech division needs to thoroughly investigated over anti-competitive behavior.
Secondly, Android is free but Google has iron grip over Android using play store. I think gated App Stores are inherently anti-competitive and not in best interests of users and developers. OS should allow multiple app stores and DoJ should go after this. Fortnite already tried to highlight this issue.
Facebook - I think Facebook might be spared as for social they are not the only platform. But given their data scandals
Apple - As I said app stores are inherently anti-competitive. Same thing applies here like Android.
Anyway, these companies can make or break business with their anti-competitive practices. As discussed in another comment, Sherman Antitrust Act doesn't apply in monopoly positions but against businesses that may stifle overall competitive behavior. I strongly believe these companies are even more scarier in telcos as they are wolves in sheep's clothing and always try to play nice.