Friendica is federated microblogging software that lives in the fediverse.
Fediverse is the collection of interconnecting servers that can communicate with each other. Mostly used protocol used is ActivityPub (later AP).
With AP users can follow other users from different servers and do mostly common microblogging actions like commenting, resharing or liking.
Total statistics of fediverse
This is the latest info about friendica according to the-federation.info website
Everybody who has the skills to setup friendica server can start to follow people from other servers. Also there is a possibility to sing up with hosted server, which has sign in enabled. That is also a good way to start exploring friendica as product.