This kind of fall in active player numbers is common, and happens very naturally as people get tired of playing the same game all the time and of the (perceived, subjective and otherwise) problems in the game. At some point, the playerbase will mostly stabilize, but even then people will likely keep trickling off. New content and events will bring back old players for a while, and sales and promotions will bring in new players (who will trickle off in similar fashion, with minority staying).
The numbers mean nothing if there’s nothing to compare them to. If you want to worry, you are free to do so, but if you want to show others how worrysome things really are, you need to give context, likely in the form of similar stats for other games. Most helpful would be ones for other small-group co-op games, but those are limited, so a few others would also be appreciated. My hypothesis is that if you were to research things, you’d find quite similar decline in most other games.
The relative amount of players is something to note, too. At the moment, we’re at over ten percent of the highest average right after launch, about the same relation in peak players.
Also some of the Steam achievements are something that can give interesting, relevant information, namely how large part of people have played the game beyond just starting up the game and trying it out. I did give some comparisons to Borderlands 2 about a month ago (a game that would be a good candidate for the player number comparisons as well): From 70k to 7k constant players. How