Growth is not a hockey stick
A certain percentage of the Twitter exodus were always bound to return. This is perfectly normal: new services always experience “scalloped” growth. That’s where an outside event — a positive narrative about the new service, or a catastrophe affecting the old one — drives a surge of new users.
Some of those users try the new service, decide it’s not worth it, and leave — but not all of them. Each event triggers a high tide of new signups, but the low tide that follows is still higher than the old level. Surge after surge, the number of users steadily builds, despite the normal ebb and flow.