From reading job ads, I think most people work in the languages you mentioned.
HN is more oriented towards python/ruby/objective C because it's demo is making relatively simple applications as fast as possible, and critically don't have legacy code.
C# and Java have moved a lot in the past 10 years. You could say that Scala is the next natural transition for Java. Microsoft have a really compelling offer in terms of Azure and .net 4.5, and have a functional language of their own (F#) if they take off.
Personally, I'm watching C++11. It is really transformative, making the language feel much more modern (even sort of python like). If it improves C++ productivity enough we may see a resurgence, although I think it does too little to make coding for parallel/asynchronous natural.
you are going to see different langs here than what's generally trending at stackoverflow purely due to the culture that surrounds hn (which is start-up based), so you're going to see more Ruby, Python, Go, ect (although you should be able to search all those tags on SO as well).
In the industry (as far as what I read anyway), it seems mobile programming and 'Big Data' are currently at the forefront.
Web vs standalone: depends on the application. If there is a need to make it web-based...then make it web-based. If there's no need then why put in the extra work/resources.
It really is a very broad question, but you want to pick the best tools for the job. You wouldn't use a spoon to saw a tree down, similarly you wouldn't use Java for a car control system. The only way to know the differences is to dive a little into each language and learn it's pros/cons.
Another vote for "It's often a matter of where you look and what you read". Like others have said, HN has a slant towards some languages - if you only took thoughts/advice from this site, you'd be led to believe that everyone everywhere uses those languages. That's not the case though - look at lots of sources.