fosstodon.org is one of the many independent Mastodon servers you can use to participate in the fediverse.
Fosstodon is an invite only Mastodon instance that is open to those who are interested in technology; particularly free & open source software.
If you wish to join, contact us for an invite.
Pixelfed is able to scale so well atm because I copied @Mastodon's feed fanout impl. and a few other things, we owe our success in part to their pioneering work and for giving us a platform to grow!