Onavo is a VPN feature that Facebook offers.

It turns out that Facebook spies on this VPN traffic.  And, once
installed, spies on your traffic even if the VPN is turned off!

Wow.

I don't think that it works on Linux (except Android Linux).

<https://www.findvpn.com/facebook-onavo-vpn-collects-user-data-even-when-it-is-disabled/>
<https://gizmodo.com/do-not-i-repeat-do-not-download-onavo-facebook-s-vam-1822937825>
<https://techcrunch.com/2018/02/12/facebook-starts-pushing-its-data-tracking-onavo-vpn-within-its-main-mobile-app/>

(My daughter pointed this out to me.)