Google's blog just published some info about Unicode use on the web. Unicode nearing 50% of the web . By Mark Davis, Senior International Software Architect. @ Source googleblog.blogspot.com
Here's their graph:
Note that Unicode is also the standard char set in many computer languages. For example, Java, XML.
For popularity of other web stats, such as wiki engine, js library, cms, see: Web Tech Stats 2010.