Hmmm ... it might have been, I'm not sure. If so, not since or during the twitter migration, as my understanding there is that mastodon.social couldn't scale well and so it was up to all the other and often new instances to take on the load (similar to lemmy's story).

Since then, AFAIU, they've put their server on kubernetes so it can horizontally scale, and of course, put themselves as the default instance in the mobile app. But as user growth hasn't really continued on mastodon it's hard to tell what the dynamics are now. Having glanced at the numbers a bit over the past year my impression is that the proportion of users on mastodon.social has been slowly growing. I haven't seen any analysis though. I did some analysis early last year (https://hachyderm.io/@maegul/110331433071884694) and maybe I'll repeat that soon.