I'd bet vastly more people have left Google (in both absolute and relative terms). Employees at both companies would have plenty of opportunities if they leave and little upward mobility if they stayed.

Paul Bucheit, Evan Williams, that same founder of del.icio.us and many many more too.

More founders leave Yahoo than Google Many founders of Google-acquired companies are still there (Youtube, Maps, ...)

Out of all companies acquired by Yahoo only founders of most recent acquisitions are still there.

Citation?

List of acquisitions: http://en.wikipedia.org/wiki/List_of_acquisitions_by_Yahoo! Founders can be found through CrunchBase Founders current status can be checked at LinkedIn

I know, it is a lot of work :)

Some cases where I am sure: Zimbra, Yahoo Groups, Flickr, Delicious