#Mastodon is kinda the open source alternative for #Twitter but there's more!
For almost all 'mainstream' (social) platform there are FOSS (free open source software) alternatives you can host yourself!
Here are some you should checkout:
Pixelfed (Insta alt): https://pixelfed.org
PeerTube (YT alt): https://joinpeertube.org
Matrix (Chat): https://matrix.org/docs/projects/server/synapse
Pleroma: https://pleroma.social
Misskey: https://join.misskey.page/en-US
WriteFreely : https://writefreely.org
Pleroma is a federated social networking platform, compatible with GNU social, Mastodon and other ActivityPub and OStatus implementations. https://pleroma.social And it has a gopher server.
Bobinas P4G is a social network. It runs on GNU social, version 2.0.1-beta0, available under the GNU Affero General Public License.
All Bobinas P4G content and data are available under the Creative Commons Attribution 3.0 license.