[nightmann]

I'm not sure what Google Talk's traffic is compared to Whatsapp, but gtalk allows simultaneous logins across many devices and does a decent job of synchronizing messages across them -- phone, tablet, computer (though there is a slight delay depending on the volume of messages it needs to sync).

[acchow]

I seriously doubt Google Talk (now called Hangouts) has anywhere close to Whatsapp's traffic. But even if they did, Google can support simultaneous login with synchronized state (and server-stored state) because it's solved a lot of these big problems in the past for other products with hundreds/thousands of engineers.