It's mostly legacy now. Many critical business systems are still in COBOL simply for the fact that they are so big and integrated that the cost of rewriting doesn't seem worth it. Writing a new system in COBOL is probably not feasible anymore, as most COBOL developers are so scarce that they can pull in a considerable amount of money for the specialized skill (similar to a Foxpro developer now). There are few to no reasons to keep a COBOL app around, but unfortunately the common reasoning is when the COBOL app is already in-place, trusted, and tightly coupled with other systems to where it's near impossible to replace. That reasoning is exactly why it should be replaced before it gets to a situation where the only hardware that runs the app has to be custom built from Ebay parts from the 80/90s.
Grab a copy of Working Effectively With Legacy Code by Michael Feathers. It is meant to deal with such situations. Even though it is focused more on OO languages such as C++ and Java, I believe it may still help you a lot.
Looks like part of it (or an early draft article) is even available for free here.
Best Answer
It appears that Ada was last updated in 2005 and there's work toward an Ada 2012, so the language itself is still alive and kicking.
As far as use, Ada isn't mandated for use in the Department of Defense anymore. Most of the work that I've seen and done has been in Java, C, and C++, but there's also use of the .NET framework and I've even heard of projects running other JVM languages such as Scala, depending on how the system will be used. There's probably a lot of code out there in Ada, so I wouldn't be surprised if there's a lot of code reuse and maintenance happening. Given the nature of defense projects, it's hard to come up with specific numbers as to its use.
There is a list of Ada projects and users, but it looks like the last update was in June 2008. There might be more recent lists out there, but I couldn't quickly find any.
If you're asking if there is new Ada development, I wouldn't be surprised if new systems are being written from the ground up in Ada. I would suspect there would be too many, but there are probably some out there. However, I would suspect that most of the Ada work out there is maintenance or upgrades on existing systems, not new development.
Knowledge in anything sells, if you can find someone who is looking for that skill set. Even if you aren't using Ada in development, I've found that knowing a particular language or framework has changed my opinion and how I use other languages or frameworks. I would suspect that knowing Ada would give you an insight into other methods to design and construct software in other languages as well.