Nerval's Lobster writes: Facebook CEO Mark Zuckerberg decided to drop a cool $16 billion on WhatsApp, a messaging service with 450 million users. It was a mind-boggling sum, even if you buy into Facebook’s argument that WhatsApp (which will continue to operate as an independent subsidiary, at least for the moment) will soon connect a billion people around the world. But it wasn’t the biggest tech acquisition of all time: that honor belongs to Hewlett-Packard, which bought Compaq for a (inflation-adjusted) $33.4 billion in 2001. Facebook’s purchase of WhatsApp comes in second on the list, followed by Hewlett-Packard’s purchase of Electronic Data Systems for $15.4 billion; Google’s acquisition of Motorola Mobility for $13 billion, and Oracle snatching up Peoplesoft for $12.7 billion. In sixth comes Hewlett-Packard again, with its Autonomy buy in 2011 (for $11.7 billion), followed by Oracle’s BEA Systems acquisition ($9.4 billion) and Microsoft seizing Skype ($9.0 billion). What do many of these highest-cost purchases have in common? Many of them didn’t pan out. Hewlett-Packard’s Compaq, Autonomy, and EDS acquisitions, for example, made all the sense in the world on paper, the tech giant eventually took significant write-downs on all three (Autonomy in particular was an outright disaster, resulting in a $8.8 billion write-off and widespread allegations of financial and management impropriety). So it remains to be seen whether Facebook can use WhatsApp in a way that justifies that monster investment. History suggests that things might not work out as intended.