The tongue-in-cheek answer to this is that they all eventually get bought by Facebook or Google. Anything that poses a threat to Facebook or Google will fold for the right price, as a simple matter of serving the fiduciary interest of the investors.
The only platforms which can hold out are the ones explicitly designed to resist centralization. They have no pitch to make to investors, no funds for promotion, but the technology is there. Mastodon and Matrix are proof.
The technology is dead simple. Facebook, Reddit and Twitter are nothing but glorified shiny message boards. WhatsApp, Discord, and Skype are nothing but glorified shiny VoIP apps. The only thing that makes them special is the branding