I can't find data for it but my prior would be that the overwhelming majority of "normal" users use Twitter primarily through the smartphone app as opposed to the web interface.
The only person I know who regularly uses Twitter says she has never visited the site in a browser and is quite sure that everyone in her circle uses the app. But that's just anecdote
No, that's not what I meant. What I found strange is the premise that most people use websites so little they'd be confused by what amounts to a URL or email address.
It's not a matter of using websites so little. It's a matter of not understanding how stuff works and most people don't. Most people open "the internet" (their browser defaulting to google), type "facebook" and then click on the first link they get. Most people don't even notice that there is some cryptic stuff at the top (or bottom) full with slashes and weird words and browsers have tried to hide the URLs for some time now anyway. Even if some people notice, they quickly dismiss it as a techie thing they don't have to understand; facebook works for them and that's all they need.
The only person I know who regularly uses Twitter says she has never visited the site in a browser and is quite sure that everyone in her circle uses the app. But that's just anecdote