Everyone knew at the time that Facebook bought Instagram because it threatened Facebook's dominance, and hindsight shows that exactly that happened. There's a huge swath of people that dropped off FB and now use Insta, but Meta owns both. It was a great move but it was absolutely anti-competitive at the time.
Is that illegal? I don't understand! Every company that buys another company buys it because it adds something to their business. It's a ridiculous claim.
[1] https://eur-lex.europa.eu/eli/reg/2016/679/oj/eng#art_85
Doubtful.
The only real solution is to implement some sort of identity management system, but that has so many issues that make it a non-starter.