I'm not here to debate you on anecdotal data (I know 5 people off the top of my head who do use it).
It has a respectable 10,000,000+ installs on the google play store and ranks high on the apple store with 270,000+ reviews. Though not comparable with the likes of WhatsApp (which I personally use and prefer), I would not dismiss it as 'not' a mainstream option. We can agree to disagree for sure.
Although it is a privacy based, (good for _some_ people) I don't see why the masses would use this if WhatsApp is there.