I know a number of big companies (like mine) are using Infinispan. The thing, though, is that people using the community version may not want their usage advertised.
Red Hat also offers a commercially-supported version of the data grid product called JDG. There you may be able to learn of other companies/projects and their data grid usage.
If you want, you can mine the Red Hat Summit/JUCon presentation archives regarding Infinispan. There are some interesting presentations that may give you some of the answers you may seek.
Note that JBoss Application Server uses Infinispan for implementation of its clustering layer, so there is a pretty large deployment there.