That's one reason … but here are a few more …. 

1. 450M active users (Source: NY Times)

2. Adding 1M users daily

3. 70% of MAU use the service daily (Source: TechCrunch)

4. WhatsApp offers users in Europe, Brazil and other emerging markets (= net new audience) (Source: Gravity/Techcrunch)

5. Nearly 200 minutes of usage each week (Source: Mobidia)

6. Facebook gets how to monetize mobile through paid advertising without wrecking the user experience. (In Q4 2013 they crossed over from 49% of revenue from mobile to 53% from a base of 945M mobile monthly active users) Source: Facebook, TechCrunch

 

Why $16B to $19B? I am not a financial analyst, but here are a few thoughts:

– Facebook generated $1.37B in mobile revenue in Q4 2013 on a base of 945M users … annualized that is $5.80/MAU (monthly active user)

– WhatsApp already generates $1/user for a chunk of their users through a subscription fee (less fee to app store?)

– If WhatsApp users can be monetized at the same value, that adds another 50% approximately in mobile ad revenue

– Facebook reported 914 minutes of use on mobile per month in 2013 (Source: allthingsd.com)

– According to Mobidia, only Kakao Talk has more

– WhatsApp is already located in Si Valley

There are a dwindling number of privately held companies with audience numbers in the hundreds of millions. Tencent (WeChat) is public. LINE belongs to Naver Corporation. Rakuten bought Viber last week. 

Kudos to the founders for not snubbing a few billion dollars.