Sun Microsystems was aquired by Oracle (java, mysql, hardware division, patents and all) for $5.6 billion. Sun could probably be said to have more 'users' than what's app, but it's a different context for the user.

Motorla Mobility was aquired for 12.5 billion.

I don't even know what to compare the whatsapp aquisition to. The valuation seems quite insane.