I witnessed something recently that points unambiguously at Whatsapp chats being not private.

Not two months ago I sent a single photo to a friend of some random MacGyver kitchen contraption I made. Never described it, just a photo with the lol. He replied lol. He never reshared nor discussed it with anyone else. We never spoke about this before or after. Two days later he starts seeing ads on Facebook for a proper version of the same. There's virtually no other explanation except for Meta vacuuming and analyzing the photo. None.