From a Blog Post by Peter Mika:
The data shows that the usage of RDFa has increased 510% between March, 2009 and October, 2010, from 0.6% of webpages to 3.6% of webpages (or 430 million webpages in our sample of 12 billion). This is largely thanks to the efforts of the folks at Yahoo! (SearchMonkey), Google (Rich Snippets) and Facebook (Open Graph), all of whom recommend the usage of RDFa. The deployment of microformats has not advanced significantly in the same period, except for the hatom microformat.
More info and a bar graph are included.
Peter Mika is a researcher at Yahoo Research in Barcelona.
A SemanticWeb.com post (where we read about Mika’s statistics) also points to a post about Facebook’s use of two microformats.