If two of your recipients interacted with a certain ad, there's a chance you have similar interests.

Combine this with the frequency of your chatting and your location (at least based on ip) and the other little bits of stuff users give about themselves, Meta doesn't really need to know specifically what the contents of your messages are.

In the mass of their users, an informer smart guess is more than enough.

Top of my head:

* Who are you messaging most (best friends, family). If they like stuff, you might like the same stuff.

* When are you messaging people (awake time > profiling)

* Messaging companies (obvious, what are you into)

Also: Where are you messaging

You clicked an ad about Product X, you're messaging your friend B from a store that sells Product X

-> Serve ads about Product X to B.

You make a post looking for a plumber, you spend time chatting with people who are plumbers in their profile - you are interested in plumbers

Example: Went to the dentist, and the clinic messaged me a confirmation message via whatsapp. Next day, I got several ads for orthodontic braces.