Yesterday I mentioned vodka to my wife in Facebook Messenger and a few minutes later she saw a vodka advertisement on her timeline. Obviously Messenger will never include end to end encryption.
OP has shown that and how FB does in fact use text analysis of messages to show adverts. You habe been a re-targeting target by correllating your browsing habits and other factors. That has been going on for ages.
edit - I meant to say even if they had end-to-end encryption then your text would still need to be decrypted so that other user can read it.
I mean, my message contents are personal data. The topics "buy a bike" and "sell a bike" are not (I mean, not by themselves), so those you can transmit to ad companies.
Now of course you need to make the detection lightweight enough that you don't need to talk to the server anymore, but that's just a matter of time. The person you're responding to, who's got downvoted, has a point (if this was his point).
Edit: I sounded like I agreed with it, which is not the case. Even if it's stored "private and securely" at Facebook, they would still be able to connect my topics of interest with my identity and contacts, which would "technically" comply with the concept of end to end encryption (only metadata leaks because topics alone are metadata, which always happens because you need to route messages)... but which still doesn't comply with my own definition of proper privacy.
This is just how marketing would sell these features to the world (end to end encryption, and yet targeted ads), but disclosing each topic I ever discuss with a given person is still private in my opinion.
I've edited my post's original text slightly, adding a bigger edit at the bottom.