The TIOBE Programming Community index is an indicator of the
popularity of programming languages. The index is updated once a
month. The ratings are based on the number of skilled engineers
world-wide, courses and third party vendors. The popular search
engines Google, Bing, Yahoo!, Wikipedia, Amazon, YouTube and Baidu are
used to calculate the ratings. Observe that the TIOBE index is not
about the best programming language or the language in which most
lines of code have been written.

http://www.tiobe.com/index.php/content/paperinfo/tpc
i/index.html

A.L.