Tiobe Software ranks the popularity of programming languages based on references in search engines. While the methodology might be debated in terms of the absolute rankings it produces, it is quite interesting to see how the rankings fluctuate over time: Tiobe has produced a monthly report of rankings based on this methodology since 2001.
In the Tiobe Programming Community Index for February 2011, the top three slots are held by the general-purpose languages Java, C and C++. Domain-specific languages naturally fall farther down the list: in this months report, R is ranked at #25, with Matlab at 29 and SAS at 30. What's interesting is the movement: Matlab is down from #19 a month ago (and #20 a year ago), whereas R is up from #26. But look at SAS, mentioned in the report's summary as having "lost much ground": it's down from #16 a month ago and #14 a year ago.
Tiobe Software: TIOBE Programming Community Index for February 2011