Well, compared to other games, it is dead. If you try matchmaking for co-op, you'll never find a game, because no one else is searching.

But there's still a few of us that do play, and you can find plenty in the steam forums.

Or go here, which is probably easier.

http://gtm.steamproxy.vip/groups/HFRR

Make some friends, invite them, have a good time.