Threema is “big” in German speaking countries - they’re usually high up the paid app charts in Germany, Austria and Switzerland and may have reached a level where (social) network effects are working in their favor.
They got a significant boost in German press and downloads when Facebook bought WhatsApp which seems to have propelled them ever since.
What’s interesting about this is it shows that while the majority “don’t care about privacy” a significant minority do care - enough to allow a company to thrive.