Obviously, antitrust laws are among the many things I know nothing about, but it still difficult for me to understand how they can allow facebook, instagram and whatsapp to be one corporation.
With respect to Instagram, it's really not hard to understand - back in 2012 (when Facebook made the acquisition) Instagram had just 50 million users and made no money. It would have been ridiculous for anti-trust authorities anywhere in the world to step in. Remember too that the general consensus among a great many back then was that 1 billion dollars for Instagram was an absurd purchase price for what was perceived as a relatively tiny little start up with a very uncertain future.
While everyone likes to bash Facebook, the growth they have achieved with Instagram since acquiring it is pretty impressive, 50 million to a billion users in 6 or so years, and it's not like there wasn't significant competition along the way from Snapchat etc.
Anti-trust law doesn't prevent any form of consolidation. As a general guideline, the DOJ is only opposed to going from 3 to 2 competing offerings to customers.
Given the existence of countless other social networks, photo sharing websites and messaging apps, it would be hard to justify blocking any of these deals, especially in the case of Instagram which was still a very young and nimble company at the time of the acquisition.
Corruption might be the explanation.
the US's legal framework for anti-trust come from the Sherman anti-trust act of 1890 and the Clayton anti-trust act of 1914 (note the dates). The jurisprudence basically holds that being a monopoly isn't illegal per se, but what's illegal is abusing consumers - traditionally the cause of action in anti-trust lawsuits are that a corporation does something anti-competitive to increase prices. Facebook is mostly available at no cost to consumers (or at least, consumers don't have to pay to use the products) and so they avoid anti-trust scrutiny.
To see it in practice, check out [1]. Some Japanese auto part suppliers were acting in an anti-competitive way and what got the DOJ so chuffed was that consumers had to pay more for cars and car parts because of the behavior.
The Yale law journal has a great expose on the tension between our 19th/early-20th century understanding of abusive monopolies and the realities of 21st century commerce [2] and basically the core issue is that anti-trust laws are powerless to stop a company from acquiring the power to become an illegal/abusive monopoly.
The EU has actually been much more skeptical/hostile of the arrangement between Facebook/Instagram/Whatsapp , and the best legal thinking on using anti-trust norms to break up big tech comes from the EU. Unfortunately, I don't know much about it and I'll invite any expert to weigh-in.
[1] https://www.justice.gov/opa/pr/japanese-auto-parts-company-p...
[2] https://www.yalelawjournal.org/note/amazons-antitrust-parado...