It is WhatsApp and Messenger users who were moving, if you need to talk to someone right now and the platform both of you were using is not working for hours, then it is not surprising people moved to another platform quickly, especially if phone calls/sms are not feasible (expensive/international/no video etc)
Few people have that much brand loyalty to delay communication with all their friends and family when a platform is down for so long and there is viable alternative available.
It is not because they ideologically cared either way, they just want a platform that is easy to use and other person is ready use as well.