Isn't this the purpose of most acquisitions?
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.