The only definition of user experience that puts Facebook or Twitter ahead of alternatives is a broad definition that includes the network effect (people I know are on it) and familiarity (it’s what I’m used to).
But in such a broad definition, I would then include things such as ‘you manipulate me with ads’, ‘you steal my attention’. This is bad UX.
In a narrow definition of user experience – how easy is it for me to sign up; how easy it is for me to share an image; how easy is it for me to share a note; there is nothing special to Facebook or Twitter in these regards.
You rarely hear someone complain about the user experience of signing up for an email provider. But that’s (nominally, at least, gmail black hole aside)
a decentralised service. It’s just because people are familiar with it and it’s where people already are. If a company came along and said – hey – for email – everyone on the planet must sign up to this one megaservice to exchange emails from now on. People would say – what a terrible idea. Because people are familiar with it not working that way.
I don’t think centralised services have better UX. All they have is a monopoly.