From the book "The road to Reality" by Roger Penrose:
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[...] String theory is also a subject that is studied by a good many physicists, but does that make it physics? This raises the issue of fashion in fundamenta physical research. Let me begin by quoting a survey carried out by Carlo Rovelli, and reported in his address to the International Congress on General Relativity and Gravitation, held in Pune, India, in December 1997. Rovelli is one of the originators of the loop-variable approach to quantum gravity, and he claimed no professionalism in the conducting of his survey. Yet the results he found certainly reflect what my own (unsubstantiated) expectations would have been. He made a count of articles on the subject of quantum gravity published over the previous year, as recorded in the Los Angeles Archives. The rough average of papers per month, in the various approaches to the subject, came out as follows:
String theory: 69
Loop quantum gravity: 25
QFT in curved spaces: 8
Lattice approaches: 7
Euclidean quantum gravity: 3
Non-commutative geometry: 3
Quantum cosmology: 1
Twistors: 1
Others: 6
