For many years after release, Freespace 2's Multiplayer system was a service known as PXO which provided lobbies and a central hub for those who wanted to practice some unfriendly dogfighting with friends. Alas though, PXO gave up the ghost in 2004 - 5 years after the game's release.
When that happened the community rallied together and worked to setup FS2NetD - the community's replacement for PXO which would provide the same services to allow players to continue using Multiplayer. In fact it was one of our own members, STR-Killjoy, who did most of the heavy lifting to get FS2NetD on its feet!
Whilst that version of FS2NetD has now long since been retired, its spirit lives on along with its replacement - "FS2Net". For more information and instructions on how to get yourself setup and into the lobbies, check out the 3.6.18 Multiplayer Setup thread over at HLP.
Whilst not many of us play actively anymore, if you're looking for people to fly with try asking around in our Discord. It could be that some of us are up for dusting off our joysticks and taking to the nebula once more!