Myspace had certain problems and probably didn't scale well, it had UX problems and management problems, combine that with the fact people wanted better granularity of privacy settings (though to be fair, Myspace was decent about it. There was a publically visible wall, but, still) and Facebook seemed to offer that, but of course Zuckerberg continually subverts the settings because he does not care about privacy (Facebook actively has switched people from 'friends only' to 'friends of friends' or 'public', multiple times), and also the fact Facebook apparently built a tool early on to import your contacts to Facebook from Myspace, but, of course, not the other way around
Google later offered a prompt in their own services, because Facebook was hoovering up contacts or... something, I forget
Zuckerberg actively wants a monopoly. He's, in my estimation, a sociopath. He got lucky, but 'lucky' also means 'internet 2006'. If you steal the Winklevoss' product (something he apparently also did with 'Metaverse'. The twins meant it in some sort of bitcoin sense, Zuckerberg co-opted it) you too can capture the literally entire global market, because the world was just coming online in those years