It has been 4 weeks ago since I've launched a new version of teamwork.tf, more geared towards community server quickplay and statistics gathering and I thought these statistics might be valuable to the comp scene as well.
The website gatherers all game server info from the TF2 game-coordinator and processes those servers run by the community into the quickplay segment, and uses all the other data for statistics:
Live statistics about TF2:
Competitive lobby/league providers: https://teamwork.tf/community/statistics/competitive
Overview / ratio of in-game players: https://teamwork.tf/community/statistics
Valve game-servers only: https://teamwork.tf/community/statistics/valve
Note that these stats only include players that are actually in a game-server, not counting bots or people idling in main menu. Also note that Valve comp is not included in the competitive stats, because there is no way to distinguish between casual and competitive valve servers.
If you take glans you can see that the average peak of comp. TF2 is ~500 players in-game for about an hour a day. If you go to the overview/ratio page you can disable valve/community players in the history graph to see the exact amount of comp. players.
Another interesting fact is the huge difference in the way that Steam reports playing users, compared to what is being observed of in-gameserver players as noticed by MR SLIN. I cannot be 100% sure about this, but I'm pretty convinced this difference has to do with the way that Steam counts, which would by done by counting the Steam accounts reporting back to steam they have TF2 open. Because there are a lot of Steam bots, and people might only have the main screen open this huge difference can be somewhat explained. Other factors might include: queuing up and farming metal (I've seen huge metal farms during my scans). At last they might count per users time-frame instead of one snapshot.
Oh by the way, I've also created some other statistics charts in the past in case you're interested:
https://www.reddit.com/user/teamworktf/submitted/