If Gmail reads private emails to target ads [1], why would people think that Facebook isn't doing the same thing? Also many companies (including Facebook) have agreed to share hashes and cooperate to "remove extremist content from their websites" [2]. It would be naive to think that they are excluding private messages from their searches. [1] https://www.theguardian.com/technology/2014/apr/15/gmail-sca... [2] http://…
WhatsApp is from Facebook and it implements the Signal protocol which offers end-to-end encryption. That's why Facebook with WhatsApp cannot do the same as Google does with Gmail. If they do, then their E2E encryption is fucked up.
You can probably trust whatsapp end to end encryption as much as you can trust facebook to protect your privacy [1][2].
[1]: https://www.ghacks.net/2017/01/13/whatsapp-security-make-thi... [2]: http://www.ibtimes.co.uk/israeli-firm-allegedly-selling-spy-...