For some reason I thought part of the reason was ex-googlers at Facebook. But it actually seems that it was somewhat the other way around. Mercurial is used at

For some reason I thought part of the reason was ex-googlers at Facebook. But it actually seems that it was somewhat the other way around. Mercurial is used at Google partially because Facebook was already using it. Maybe someone has a more concrete timeline of the cross pollination of VCS between the two.