@freemo I moved to Mastodon due to the recent Whatsapp privacy policy which extends its services to Facebook and its policies. And Instagram is also following suite in becoming very invasive. Although I already used Signal before, now I finally have many of my friends moving to Signal from Whatsapp.
By the way Whatsapp is the most popular messaging app in the Asian Region with almost 340 mill users. Its a big number, and hence many of them want an open source alternative for everything now.