That is not the case at all, at least in our business.
Anyway, I started playing with Discourse to migrate a community from Facebook back in 2014, and also did a successful one here at the company for a big customer, so it’s not like they are a myth. Also, both migrations were very successful.
In both cases, the most important thing is what is your capacity to pull people to the new place. In both cases, we have a large number of benefits to members to migrate over so they came in droves and the Facebook community was abandoned organically when everyone moved.
We keep repeating this point over and over here, you can’t simply create a Discourse instance and expect people to come over, you need a whole go to market strategy and several compelling reasons for users to migrate. If you don’t have that, you are doomed to fail.
Here it is facebook-migration