I would prefer not to federate with them. Their track record is abysmal and the privacy invasion is horrendous. One of the people over on Mastodon recorded more than 200 queries from the app within an hour of installing it - they are determined to scrape every last detail of your life into their advertising database and, well, fuck that.
Mastodon.nz announced a week or so back that they have no intention of federating with them and I found that to be reassuring: we're all here because we specifically Do Not Want a corporate body throwing adverts and algorithms at us.
It wouldn't surprise me if blasting the fediverse with millions of users is specifically designed to weaken smaller instances, just to oh-so-graciously offer their users and 'safer, more stable' place to set up.