(WhatsApp only had, like, 50 employees when FB bought it for $19B, as a bit of evidence that headcount isn’t necessarily a measure of value.)
I believe intention and behaviour matters much more to antitrust than simply continuing to be a dominant market leader by smartly staying on top of what the public wants. Google search doing horizontal integration into Android and Chrome to cut off competition's market entry points at lower levels is far more plausible antitrust narrative IMO.
Since it didn't happen it's pretty moot to the discussion. M&As with massive market consolidation like that get challenged all the time in courts so it's hard to say what would have happened then.
FTC investigated the Instagram purchase in 2012 and chose not to interfere.
So failed acquisition doesn't make buying Instagram inherently an anticompetitive move, just maybe can be spun as their strategy at the time, but a decade later the market still has plenty of players so it will be a tough case to make. Although even a failed case gives the government leverage over Meta with threats of future ones so they might not care.
The FTC investigation specifically had a condition that they would reevaluate the acquisition in coming years. This was widely expected to be done, and everyone knew about it at the time of the acquisition.