Instagram did send a little notification to their user base.
But here’s the thing - only approximately half of mobile users subscribe to push notifications. Then only 3.4% of notifications actually get clicked.
So assuming 100% of the users who clicked the notification downloaded Threads, they still would have only got a ‘measly’ 8.5M users.
To kickstart a social media of this scale this just isn’t good enough. And Instagram knew it.
So they focused on building viral hype and incentivising users to signup asap.
First, each user was numbered based on how quickly they registered for the platform.
Second, each Instagram user's followers only automatically get added to their Threads account if they register before their followers.
So no-one wanted to be late to the party. And as a result most users got to Threads through word of mouth or some form of media.