There is Battle for Sinai as an active, persistent server, but this is mainly PvP. And it's brutal. There's also Veterans Gaming, but I've not tried that yet.
I don't understand why people always ask for a server to play on ---- as you can easily host a game yourself. I'm on a measly 2MB down, 0.8MB up and I can host for a 4-ship, maybe a bit more. If the objective is to meet someone who will take you under their wing, join a virtual squadron. If the objective is to meet up with someone you already know in order to fly, host the server yourself.
While you can refine your techniques offline on TEs and maybe even fly your own campaign, I do agree that there is nothing that compares to flying with a human wingman, but like I said, you don't have to go server-hopping to do this.