Rare Monster Hunting & Tracking; There's a linkshell for this (bounty hunters)
Savage, Ultimate, Extremes challenges (synced); There's a discord server for this (Crystal raiders, I think, and UCR for ultimates)
General Chat of the Server (mature & polite); Either novice network if you have it, or the Crystal datacenter discord server,
Eureka & Bozja hard content. Discord server (Crystal exploratory missions or Eurekan explorers.)

Unfortunately the in-game social options aren't really used much. :T