Pretty sure this is just the result of Letterman merging two of his Pages together: https://www.facebook.com/help/249601088403018
What's the point? To have thousands of irrelevant users? It's going to be even more difficult to determinate the follower's profiles.
The title of this post is a bit misleading. The example given here isn't really a stretch, since the audience for the first page is very likely to follow the new page anyway. Is there any evidence at all that any old FB page can just add followers on its own?
In that case it's just public figures or brands who paid for it apparently (Netflix)
David Letterman has a page, he then created a new one for his upcoming Netflix show and Facebook apparently let him make all his previous page fans into fans of the new page. Automatically.
Which is probably not possible for normal pages and users.
I’ve had to do this once before for a client. Merging duplicate pages was available to anyone whose account has administrator privileges over a Facebook page. I had to fill out a form and wait a while for approval, but neither myself or my client had ever paid for advertising through Facebook