It seems pretty clear cut that it’s monthly active users when Llama 3 is released.
> If, on the Meta Llama 3 version release date, the monthly active users … is greater than 700 million monthly active users in the preceding calendar month …
If that’s not true then the free license applies to you.
Presumably megacorp's laywers are engaged with you doing due diligence before the acquisition, will be looking into this, and evaluating the license. Maybe they have prior licensing agreements with Meta, or plan to replace your use of Llama with something different, who knows.
OTOH if you are being acquired by Elon Musk, then there may be no due diligence, he will tear up any existing license agreements, spend the next year bickering with Meta on Twitter, then be sued to comply.
> If, on the Meta Llama 3 version release date, the monthly active users … is greater than 700 million monthly active users in the preceding calendar month …
If that’s not true then the free license applies to you.