recently mentioned the httparchive stats for 'Sites with Custom Fonts' in another topic. To me that numbers looked too good to be true (48% percent of sites using custom fonts), so I had a quick look at them:
According to httparchive's January data 232,591 out of 483,391 sites use custom fonts which is measured by counting all domains that include requests to eot, woff, ttf or otf files (this does not include domains that use web fonts by embedding them base64 encoded). What they do include of course are icon fonts, since there's no way to filter them (except by knowing all filenames of all available icon fonts).
I tried to exclude icon fonts by removing the most common ones at least (Fontawesome, Glyphicons and so on), by counting all domains that just use icon fonts (but no other font) resulting in 174,605 domains/sites which lowers to percentage of 'Sites with Custom Fonts' to 36%.
Other probably interesting stats are:
For »serviced fonts« Google is top 1 with serving nearly 88% of all requests.
Services like Wistia (an embeddable video player using OpenSans as UI font) or Tumblr serve more font requests than Fontdeck for example.
I'll compile some more stats and publish them here if someone is interested in them ...