UPDATE - new location: http://dgstats.spikx.net Just fyi, I started tracking the player numbers (and Steam stats game rank) for Monday Night Combat PC on dgstats.insidesupcom.de. For anyone who is interested in these things
No, it's the rank on the steam stats page: http://store.steampowered.com/stats/ , based on the current amount of players playing. Right now, Dota 2 is on place #1, Team Fortress 2 on #2, etc.
This graph itself worries me. Also it seems like the highest point is on a Thursday, which is when the updates come out. That makes sense, no surprise there.
The negative slope from eyeballing it. It seems like the base is dying, but I could be interpreting the information wrong.
Technically you are right, but you seem to be lacking some experience with stats like these. What exactly is your expectation? The constant trend over a longer period of time for any game is always down. No game will ever have a steady upwards trend. The only time when player numbers rise higher than usual is after its initial release, after updates, sales, new content, expansions, etc. You can look at the other stats on that page for comparisons with a few other games.
May be due to Meet The Pyro, since the the TF2 gaming community probably strongly overlaps with the MNC one.
I have moved the stats to a new location and I am also using Highstock now: http://dgstats.spikx.net It's a bit superfluous though, I recently found out, that there is another, more general Steam stats tracking site: http://steamgraph.net/ Although I think with Highstock I think mine has a better usability .