More that it is the central nexus of the federation. I am the type of person that likes to listen in the middle of the conversation, so mastodon.socal positioned in the way that it was ended up being the centre of many discussions over the over all the network and was very active.
Pixiv's Pawoo is becoming another one of those simply by the sheer volume of users on it, and the discussions over it tend to be VERY active but I don't read Japanese to save my life so what I can get out of Pawoo is going to be rather remote.
Discover-ability didn't really enter into the migration from Mastodon.cloud and I was rather happy with the instance, all things considered. It's just that mastodon.social was as the centre of it all and to be honest, the opportunity arose were I could register with it after... so I jumped.
On a side note:
I will admit that discover-ability will be an interesting problem for mastodon to attempt to solve. The search function seems rather basic (I mean to be honest, there should be an option to limit the search results to instance or the whole federation [as said instance sees it as]), and with mastodon decentralized the way that it is, it will continue to be.
I think that discover-ability will more be solvable with a organized community. In that big hub instances (like Pawoo, Mastodon.cloud and Mastodon.social) will help solve that problem, by making it easier to find people that centre in those hubs