Play store app updating process

I have recently updated an app (specifically WhatsApp messeneger) vía play store and some friends of mine can't see the new version on the store yet. How are updates managed in the play store? When a new app version is released (and I get the update available notification in the store app), is it released instantly for all devices? Is the release staggered depending on device models? Or depending on the google accounts, geographic locations, or some other criteria?