Which is a bit tragic for someone like me who lives in a place where I hardly know anyone and is desperate to talk to the other people sitting alone in the cafe
1. Facebook owns WhatsApp and uses it to collect data about people, such as who they communicate with, how and when. They also know about many of the websites you visit and what you do there. They know everything you do on Facebook, Facebook Messenger and Instagram. They buy mountains of data about us from other sources. By analysing all of that data they can probably do a reasonable job at guessing the content of your WhatsApp messages.
2. WhatsApp tries to get every user to accept the option to backup messages and photos to Google Drive, where they sit unencrypted and accessible by Google. Even if you reject that option yourself, your correspondents are likely to have enabled it (if only just to stop WhatsApp from nagging about it) and so your messages are available for Google to read. Example of why this can be bad: https://www.vice.com/en/article/zm8q43/paul-manafort-icloud-...
3. Google Photos asks WhatsApp users if they'd like it to back up their WhatsApp photos. Even if you reject that option, your correspondents may have enabled it and so your photos are stored online unencrypted and accessible by Google.
4. Why should we limit what Google and Facebook know about us? Google and Facebook influence our behaviour for the benefit of their paying customers. Their computer systems are too powerful for our minds. They work against us, not for us. Companies like Facebook will come to be seen like tobacco companies, except that the harm is as from mind altering drugs. There is a documentary on Netflix called The Social Dilemma which explains this well. The polarisation of societies and the spread of conspiracy theories are some of the effects. The only defence is to disengage.
