I don't think it's exclusively one that follows the other. Users push for content migration, and content encourages user migration. However, there's not much draw for users right now to move away from Facebook, so we're more reliant on content to make the shift.
If content follows users then how did Facebook develop such a big user base? Every single Facebook user chose to create an account there. Why? Because of the content they knew they would find there.
"Follow the users" is a good way to decide where to do some content marketing next week, but it's incomplete if you're looking at how the overall information ecosystem evolves. You have to account for how new services attract users, in order to understand what established services are doing to try to keep users.