O'Reilly Radar wrote about the 3-year programming language market share trend based on computer book sales. It shows some slow decline of Java and C/C++, and the continuing rise in market share of C# (especialy after .NET 2.0). You can see how Ruby's sharp ascent follows the introduction of Rails, and that PHP's fortunes reversed before book sales showed that web developers in search of rapid development languages moved over to ROR (and Microsoft's ASP.Net suite of technologies).