Facebook and WhatsApp users can’t make money (last I checked). I think it depends more on the service generating money from users and using that to advertise itself

even the “I use this service because my friends do” argument doesn’t work because kids go to new services their parents don’t use (and their future children do the same). so I think social platforms can’t last longer than one generation*

there’s always room for new services, and it’s primarily a matter of advertising or otherwise raising awareness

*I’m unsure how or if this applies to anonymous platforms like Reddit. the point of Reddit is discussion instead of connecting with friends. so maybe it will survive longer than one generation. and possibly it’s less popular because fewer people are interested in discussion