ARC, VDC and others publish market reports for automation markets. They cost a few thousand each for the full reports. PLC marketshare numbers vary from region to region.
Allen Bradley got popular for a long time but in many areas they have definitely lost share and I seriously doubt that they have more than 30 percent of the US market share. Fanuc makes great stuff, Siemens does too and both blow AB software completely out of the water as far as I have seen