The key thing is the same as in the coronavirus pandemic, it's exponential growth - think of Facebook as the virus, Myspace as the immune system and word-of-mouth being the infection way.

The problem with all the cases you mentioned (and the current fight of Facebook/Meta vs Tiktok) is that the established entity has become entrenched in its ways and too unflexible to adapt, so competitors exploit the weakness and eventually take over.