Depending on your threat model, yes. As far as I understand they cannot read the content of your messages, however:

- by default they'll upload backups of your messages to Google cloud.

- (more importantly for me) I really don't trust Facebook to:

1.) not abuse metadata

2.) not mess up security somehow

3.) stand up against whatever oppressive regimes we might see in the future

In addition I don't want to make the Facebook ecosystem any stronger if I can avoid it.

A nice bonus is that I strengthen the network effect of other ecosystems when I use them by default.

No. Facebook bought them.

No, in response to things like WhatsApp the Australian government just passed a bill enabling it to force companies to create backdoors to encrypted communications.