Most RP I've seen is centered around Balmung, but I've also seen a fair share of people RPing on Jenova, Faerie, Mateus, and Siren (all Aether).

Since Balmung got closed for transfers, I've seen people on Mateus being the loudest at directing people to their server. There are multiple Free Companies and Linkshells up on the RPC, which I'm sure doesn't contain all of them. They've also got multiple discord groups.