Some historical statistics about the use of formalisms for modeling automated systems since 1800. Here is a chart about how frequently a particular name of a modeling formalism was used in literature. I’ve picked Automata, Petri nets, Process algebra, Statecharts, and UML.

You can get the full chart (and add your own favorite formalism) using Google Ngram. What I find surprising is that Petri nets were at some point as relevant in literature as automata (which have been discussed since the 1800s already). I’m not surprised that UML peaks all of them by far. On second throught also UML’s decline is not suprising as the hype returns to normal levels. What I *do* find surprising is that process algebras are much less referenced in literature than even the very particular, though successful, technique of statecharts.