Most of me wants to say no. Only for two reasons.

One, WhatsApp would be in kindergarten if it was a child, and its entire staff could fit on a Greyhound bus. Seeing them valued at the same market cap as a company like Sony makes my brain hurt. Oh, and they make about as much revenue as a bag boy does at the local Kroger.

Two, I’m jealous. I’ll admit it. How can I not be? A valuation of around $500-million per employee is any startup’s dream. If our company hit 1/10 that valuation I’d have a grin glued to my face. Fortunately, I’d make enough so that I could afford surgery to restore my face to normal.

But after I thought it, I don’t think Zuckerberg is insane. While there’s no revenue model in the world that can justify that price for a service that barely brings in revenue (and certainly isn’t profitable), I think that Facebook was more likely to be asking themselves, “How much is it worth to keep someone else from buying WhatsApp?”.

Yes, Facebook is the world’s largest social network and is already a bit more Orwellian than most of us would like. But it’s no longer the place to be, especially among teens. Facebook even admits that:

Just a year ago, 42% of teens surveyed told pollsters they preferred Facebook to all other services; by spring, that had fallen to 33%, and now stands at 23%.

In other words, Facebook can’t assume that younger people will continue to see its service as the de facto place to share selfies. Teens have Instagram (which Facebook bought), SnapChat (which they tried to buy), and WhatsApp (bought). And let’s not forget Twitter, WeChat, Line, and Viber (each with at least 200 million users) competing for market share. With over 450 million users on WhatsApp, Facebook would take on a huge penalty should it fall into the hands of a Google or a Yahoo!.

Facebook itself only took a handful of years to go from dorm room project to international behemoth; what’s to keep another social network from displacing it in short order? That full discussion is probably worthy of a separate blog post, but the short answer is not much. At this point, Facebook has to be just as defensive (by keeping other large networks from coming under the control of a competitor) as it is offensive (by continuing to pursue both user and revenue growth).

Is $19 billion the right amount to pay for that defense? We’ll never know for sure, but if it keeps Facebook relevant with teens and holds off a challenging social network from displacing it for a few more years, the answer is yes.