I've been monitoring the user count for DC:U since it launch and the peak traffic is current sitting at about 6300. 
 
Since you have to be online to play an MMO, this means that since Tuesday a minimum of 6300 people have bought the game on Steam. It could of course be more since they are not all online at the same time, but if you consider that it is currently the top selling Steam game, Steam sales cannot really be all that great.  
 
Now the volumes for something like Black Ops were around the 85000 mark at a maximum around its launch which suggests that higher sales figures are possible, but this is also for a game that sold a crazy number of copies on the consoles. 
 
I guess there is a reason that Steam does not make its sales figures public. Still a great platform though.

Everyone usually waits for weekend sales and buy games that are actually worth something the week it's released.  DC universe is not that game.

Steam only accounts for like a third of game sales, probably less for an MMO, and that is just peak traffic at one time, considering different time zones all over the world, the minimum may be nowhere near the actual sales.  Also, many people are still waiting for it to DL and get running.

Not surprised sales aren't that great because it's a pretty poor MMO.

Console sales for this game are high because its the first highly marketed MMO for them while on the other hand pc  gamers have seen endless waves of MMOs launch, be bad, and fail.  
Also, you  trying to use this to measure steam sales makes me think you go to a special school. Do you?

With around 40-60k people playing BLOPS, and many people swiching to Steamworks....Steam's doin well.
 
Think of it this way; if steam was doing poorly, what about the compitation? They would be homeless right now.

" Think of it this way; if steam was doing poorly, what about the compitation? "

The "currently playing" stat is only a small percentage of total users. Take CS:S and Team Fortress 2 for instance (also games that must be played online); it's common knowledge that the community is easily in the hundreds of thousands or even millions, but everyday the number only shows about 20-70K for each. That means the number of people currently playing is typically less than 10% of the total that bought it. Judging from those numbers, my guess is that no less than 100-200K people have bought DCU in total, which isn't bad for a couple day's sales of an MMO.