Permanently enjoin the acquirer from showing ads or sharing data. Heck, require operating independence. Just like any other transaction, terms like these are negotiable. Maybe those reduce the purchase price by $3 billion, or even $5 or $10 billion, but the founders would still be multi-billionaires and they’d also see Whatsapp survive the way they envisioned it. Best of both worlds.
(And if the buyer totally walked rather than reducing the price, at least you’d know their true intention.)