W3C claims that
XML is one of the most widely-used formats for sharing structured information today
Does anyone know if there is any corroborating evidence (studies, reviews, etc.) for this statement?
RSS, Atom, SOAP, and XHTML are all XML. XML-based formats have become the default for many office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork. XML has also been employed as the base language for communication protocols, such as XMPP.
XBRL is now the global standard for exchanging business information - used for tax filing, regulators, stock exchanges etc.
DocBook, ePUB, GraphML and MathML, OpenDocument, SAML and many others are all XML formats.
So to answer the "is it one of the most widely used formats" - absolutely. From the evidence I have available it appears to be the most widely used, but I don't have enough information to categorically state that.