it is by far the most used with Scholarpedia a distant 2nd “with only… 5% of the number of papers referencing Wikipedia.”
All other free online encyclopedias tallied less than 50 papers referencing them in 2011.
Citizendium: “an English-language free encyclopaedia project launched by Wikipedia’s co-founder.” Knol: “Knol is a Google project including user-written articles on a range of topics.” PlanetMath: “a collaborative encyclopaedia focussing on mathematics.” Scholarpedia: “peer-reviewed open-access encyclopedia, where knowledge is curated by communities of experts.” Wikibooks: “a free library of educational textbooks that anyone can edit.” Wikipedia: “a free, collaborative, multilingual Internet encyclopedia.” Wikisource: “Wikisource is an online library of free content publications, collected and maintained by the Wikisource community.”
Data from Scopus. Reference from: Research Trends (bibliometrics newsletter) Issue 27, March 2012. “The influence of free encyclopedias on science”, by Sarah Huggett.