It is not a company, but a federated network – anyone can run their own server and interact with the all other nodes (much like email). The software is open source and is a community effort, although the lead developer got a grant [1] that allows him to spend time to improve the project over time.
Looks like Patreon if you read the web site. And Liberapay, I guess.
It’s an open source “host your own” software, I don’t think they are trying to monetize beyond paying a living wage for the developer.
1: https://github.com/pixelfed/pixelfed#pixelfed-sponsors