Facebook has about 2 billion daily active users, and Reddit has about 108 million daily active users. This is more than 50% of the active discussion forum market, so I don’t believe that only one in 1000 forums originated on Facebook.
No one really “migrates” from a FB forum because they have too much content locked down. For my own group of 15K users I wouldn’t be surprised if the Discourse forum only has 100 members after the first year. It’s going to take a lot of time and patience to develop a strategy to work people into trusting and using Discourse. So I don’t view migration as being a) possible or b) fast.
Long-term, I intend to keep both forums and would not want to lose FB’s enormous network effects. How to coordinate content across forums is going to take time to work out. I just know I need something like Discourse to act as a reliable repository for information, because Facebook will just randomly wipe out everything for completely arbitrary reasons, without cause.