There are currently 2 replacement server implementations, the first was done in Rust ( https://github.com/steamroller-airmash/airmash-server ) powers (I think) all the US servers. It is missing some features like upgrades.
For that reason most of the 'life' is on EU servers, despite the increased latency for US players, which are using a newer and slightly more complete Node.js implementation ( https://github.com/wight-airmash/ab-server ). If you're trying it out, be sure to check out EU.