Facebook's Cross-Language Network Library
XML was not designed as a "generic data-format"; it was designed as a "better SGML", that is, a document format. In fact, it is not a good data format, as can be seen by the contortions involved in adding a type system (essential to a general purpose data format). Which still doesn't work, by the way.
Instead of relying on common, generic data-format such as XML
Besides which, designing your own data format, while requiring some care, is not exactly a Herculean labor. If they would just add product and union types, I could see using this.