Interesting, but you've got to factor in the fact that C# is a much more specific search term than Ruby...
C# searches are probably mostly about C#. The Ruby result would also contain all the searches involving "Ruby" that aren't related to the language at all.
As someone who codes in C# during the week, I would much rather play around with other languages on the weekend to gain new insight and motivation. If Ruby was a popular 9-5 language, I would expect to see a similar drop over the weekend.