It's possible the answer is more prosaic than conspiracy, and technical in nature. If FB Messenger uses a central server for discovery only and communication is

It's possible the answer is more prosaic than conspiracy, and technical in nature. If FB Messenger uses a central server for discovery only and communication is strictly Peer to Peer, as is the case in a number of messenging apps, then there is nothing to wiretap without altering the FB Messenger client to copy received texts and phone them home. And that would add a backdoor that would make it less secure for everyone. I found a link from 2019 that indicated FB Messenger uses the Signal protocol (https://en.wikipedia.org/wiki/Signal_Protocol https://en.wikipedia.org/wiki/Signal_Protocol) for their encrypted chats using the mobile client, so that makes the P2P theory more plausible. An interesting experiment: set up daily FB messenger chat with a buddy and discuss in depth some product of narrow class of products you don't usually search or discuss (Nike shoes for example). Do this for a couple a weeks and keep track of the ads online that you and your buddy see. Did the number of adds for shoes or Nike shoes go up?