Try
https://www.x-flightserver.net/It is a free online network for flightsim pilots using X-Plane, FSX or P3D, has changed to an FSD server which is reachable via swift.
I have tried it with Flightgear and Swift, it is possible to get on the server. The team is very nice and welcoming, though you do have to sign up. I have the impression, Flightgear pilots are welcome.
I haven't had the opportunity yet to see whether meeting with pilots of other sims really works, I shall look further into it and report.
Oswald