Measuring programming language popularity

TIOBE Programming Community Index based on number of skilled engineers world-wide, courses and third-party vendors
Python, C, C++, Java, C# 

RedMonk Programming Language Rankings Rank on Stack Overflow by # of Tags  and GitHub by # of Projects

JavaScript, Python, Java, PHP, C# 

Stack Overflow developer survey JavaScript/Rust

New Languages: Apple Swift, Google Go, Mozilla Rust, Gnome Vala

PyPL PopularitY of Programming Language tutorial

Hub Reports: GitHub Languages

Trendy Skills category, salary, ranking, ads

The Transparent Language Popularity Index at SourceForge search engine

Language Breakdown in Open Source OOS open-source software

State of the Computer Book Market 2008, part 4 -- The Languages

State of the Computer Book Market - Mid-Year 2009

Programming language popularity by the number of postings in Usenet newsgroups