In the interest if examining relative interest in programming languages on an aggregate scale, I decided to refer to trends.google.com.
This first chart compares C++, C#, Perl, Python, and visual basic. The notable surprises are the seeming lesser interest in Perl, Python, and visual basic. Since VB is a Microsoft only language, and frankly C++ or C# can do anything VB can do, that can be explained. I was expecting more interest in Perl and Python however - I wonder why this is?
You may have noticed I left Java and PHP off the list, here's why:
Clearly the interest in PHP and Java is much higher than the other languages. Referring back to: Where are the open source jobs? - I'll comment that an individual might be more interested in the probability of a higher salary on Java vs. PHP. Certainly I wouldn't advise locking yourself into one vendor with Visual Basic.
Based on this totally unscientific (yet valiant effort to stick to data & trends) approach, I feel confident declaring that open source programming languages are at least the equal of closed source ones. If anything, the data indicates that open source programming languages dominate closed source ones.