You can read more about initial complaint and following the trial here: https://www.bigtechontrial.com/p/zuckerberg-on-the-stand-the...
Another purpose of acquisitions is to acquire a new capability that you could not do in house that you do not already compete on.
I don't think any social media consumer is lacking choice.
The question of whether society should allow companies to perform anti-competitive actions should not be “will we be left with enough choices?”, but should be “is this an anti-competitive action?”
No, it's not. It is not normal or usual. Not even in the tech industry.
Many things have been established.
It’s not fine to do them to create, expand, protect, attempt to create, etc. monopolies.
So essentially, you can be anti-competitive only to the point where you’ve been too successful at it, then it’s bad.
Usually due to either a monopsony/cartel/monopoly which controls most of the market doing it successfully.
If the companies in the lower 5% of a market price fix or the like, no one usually cares. Even 20%, usually.
The Sherman antitrust act speaks about ‘restraints of trade’ because it has to actually restrain trade, which requires a significant degree of control - which a successful/actual monopoly, monopsony, or cartel can do.
Technically, even attempting to do it is illegal, but going after every company that tries has a bit of the same feel as locking up every single toddler because they took a swing at someone or threatened them with their cute little stubby kid scissors.
It’s a waste of resources, not in anyone’s interest, stops behavior most people would consider necessary/healthy to some degree, causes much worse problems than it solves, etc.
On the other hand, locking up a successful serial killer is just good public policy.
The difference between the two is more a matter of the success and effectiveness of their tactics, not really intent.