Why do Whatsapp doesn't sell ads?

I'm a Whatsapp user for Android.

I've already paid for my second year in the Q2 of 2011.

At 2011, the price for this app was "FREE for the first year, then 1.99 USD/ year after."

(the same price was valid for the other operating systems, except for iOS, the price is 0.99 USD/ year since the beginning)

I forgot at which times exactly, but I saw a picture that the price has changed into "Free for the first year, then 0.99 USD/year"

My first thought is:

Free = Ads
Paid apps = no Ads

But strangely, I've seen that in my boyfriend blackberry, he still can use Whatsapp normally, even tough the one year period had passed, without any single ads. (FYI, he hasn't paid).

So, how it is possibly for Whatsapp to survive?
Actually, I don't know.
Maybe their contract with Nokia or the payment from the early user of Whatsapp is more than enough to support all of us.
Or maybe there are still people that are willing to pay if they are satisfied (Me?).

I like Whatsapp, and I want to support them.
If you are using Whatsapp, try to read "Why we don't sell ads" - Whatsapp
If you are agree with their idea, maybe you could support them by sharing a little bit of your money. :)