Note that raw user numbers won't really cut it; if 1 million people left Twitter, it's possible that 500 000 of those people joined and 500 000 new people joined; the user numbers would look like everyone returned, but really only 50% did in this hypothetical.
Github is actually down and I can't push code right now, it's blocking my business which is unacceptable. It's a broken tool.
On Twitter, it's mostly the networking effect, you want to be where peoples are, and unless a large chunk move at once, you are stuck there.
On Github, it's the technical cost of migrating, the git part is trivial, but tickets, PR history and CI/Integration are another can of worms.
Projects which moved away, moved away for good, at most only leaving a synchronized mirror.
and this assertion is based on ... ?