First of all, I doubt anybody can give a definitive answer -- there's just no way to tell exactly how much any particular language is really used. Nearly anything you can measure is a secondary measurement, such as how many people are advertising jobs using that language. The problem is that this tends to show relatively new languages as dominating to a much greater degree than is real.
That said, my belief is as follows. At one time, C++ was the hot new language on the block, and there was a bubble when it dominated the market. That bubble deflated quite a while ago. Since then, use of C++ has been growing on an absolute basis, but the market has been growing (quite a bit) faster so its shrinking on a relative basis.
There are a couple of reasons this doesn't show up in most secondary measures such as job advertisements though. A couple of the obvious ones include:
- Many teams producing C++ have now had years to "settle in", so the turnover rate is relatively low.
- It's now well established where it's used, so positions tend to be filled by internal promotions.
There's another effect I almost hesitate to mention, but it's true no matter how little a lot of people like it: there are both programmers and managers who are more excited about "new" than effective. This leads to a large group of wannabes who are constantly on the move to the latest and greatest "technology" (whether that happens to be a language, framework, platform, or whatever). They get a job, loaf (or worse, actually write some code), then move on to their next victim...er...employer. They cause a lot of "churn", and inflate the number of job advertisements, but produce little or nothing of any real value. That group moved from C++ to Java a long time ago, and have long since moved from Java to C# to Ruby on Rails to Hadoop to whatever the managers are excited about this week.
Lest I sound excessively negative, I should add that along the way, a few of them really find something they're good at, and (mostly) tend to stay with that. Unfortunately, for every one who does, there are at least five more new graduates to join the throng...