Also, fb already have an ad business. They don't have a b2b software business. If Salesforce had bought WhatsApp, they might be interested in the b2b idea. They probably wouldn't be into the advertising idea.
Keep in mind that Facebook's existing ad business is 100% B2B as well and they don't use an is school Oracle/IBM/Yellowpages sales org for that. Selling API access to WhatsApp would not necessarily have to be different.
Facebook already having an ad business can also be used as a counter-argument: cannibalization. Some of the ad-spending going to WhatsApp would definitely be spent on Facebook instead if WhatsApp remained ad-free. But I can't imagine that they are not fully aware of this (even Hanlon's razer must have limits somewhere), so what remains is that they consider the expansion of addressable ad audience more valuable than other monetization strategies plus the massive hit the WhatsApp brand will take from ad introduction. And because those ads would be almost worthless without the targeting ad customers expect from the Facebook brand, the whole thing is like an indirect confession about the existence and quality of shadow profiles describing those who are not registered with the mothership. Well, that group is about to shrink...
