The MP community still is quite active, although you shouldn't expect to be able to join an ongoing battle at any time. Especially cooperative play doesn't lend to this style of massive brawls anyway, and I daresay that SB Pro rewards decent mission planning more than most other games that are out there.
So it's more like setting up a date in advance and then you gotta be there. This certainly is less convenient than, say, WoW or other massively multiplayer online games. Then again, anyone can set up a session at any point, you don't have to rely on a specific game server or anything.
About 30-40% of all SB Pro players are from the US or Canada, most of the rest from Europe. So if you can accommodate these time zones it should be fairly easy to find someone to play with. There's a TeamSpeak channel for spontaneous meets as well.
I think it's best to ask the players directly, and you'll find them over at SteelBeasts.com in the multiplayer forum.