Pleroma
Lightweight Mastodon alternative. Elixir/Phoenix, built 2017. Lower resources per instance (3-4x more efficient than Mastodon for the same user count), good for small-scale self-hosting or resource-constrained VPS.
Mastodon-API compatible — mobile clients (Tusky, Ice Cubes) work. Lower adoption than Mastodon but a tenacious community. Politically mixed: Pleroma instances early attracted edgy/4chan-adjacent users → many mainstream instances defederate Pleroma instances on sight. Akkoma is a fork (more active 2024+) with stronger moderation tools, defederation by default against known problem servers. Practical choice: Akkoma > Pleroma for new self-hosts.