That would not interest me at all. I want some control over who registers and posts. If they want to participate on my site, they'll register.
Yee Gods - having any member of facebook/twitter post what they want?
Of course not !
You can still control the register process.
Moderating all new registering request , or let them pass by default . That's configurable. (even judge by IP range)
And , you still get their email . In fact , they are truly a member in the forum , just their data (name/email/...) are 'imported' from facebook. (Because FB has already 'identified' their emails)
So that they don't need to remember another password (in this forum). They just need to click a 'facebook login' button to login.
I think most people now login FB much more than a ubb forum.