A social network with (or without) this essentially is reinventing the wheel of the whole Internet which is made of HTML pages (which are replaced by personal pages on a social network), RSS/ATOM (which is replaced with in-app notifications), e-mail (which is replaced with in-app direct messages), Usenet (which is replaced with in-app groups) etc. Everyone theoretically could just have a personal www page on their own domain instead of a social network account.
Oh, and HTML never needed CSS to be not ugly - that's the browser default settings which make unstyled (styled with default styles) HTML pages look this way. We always could just change the default fonts/colors or apply a userstyle for more complex things.
Once general population joined to the Internet (where in the past there were only nerds) the demand emerged for it to become much easier and prettier and the corporations responded offering the features pople want taking their freedom, privacy and independence in exchange (which an average person doesn't mind, all they ace about is that being "for free"). We could just build better apps (browsers with sensible defaults, intuitive e-mail and Usenet clients and web servers), but we still lack them because nobody wants do serious work for free and make great products which would suit everybody ather than themselves.