I'm from Brazil. You see small local businesses, with hand painted signs, where the phone number is followed by the whatsapp logo. Not even the name "whatsapp", just the logo.

It's scary how they took over communications over here.

"Whatsapp: The Orkut Killer"

Yes. You are totally right. And the funny thing is that for many Brazilians who can't speak english WhatsApp became "ZapZap" (much easier for Portuguese speakers to say) or just "Zap".

I have never used Whatsapp in the US. The sole purpose I have it on my phone is for communicating with AirBnb hosts when traveling abroad.

It is amazing how Whatsapp rose everywhere but in the US.

You don't understand. People don't associate that number with a phonecall, but with Whatsapp.

Exactly, people don't associate the number with phones. And in my case, my "whatsapp" is a Canadian phone number. I don't even know the number on my Brazilian phone.

Not necessarily. My phone number when I go back home (if anyone wants to call me) is a different number from my whatsapp number (which is my UK number, where I live). When people want my Whatsapp number, I generally give them my UK number, but I don't expect an SMS or a phone call from them because I don't even use that SIM card back home. I have another local SIM card that I use, and I also give that out as my "phone number" in case, you know, somebody wants to call.

Did they ever fix the massive security hole(s?) in whatsapp?

Apparently they partnered with the TextSecure people https://whispersystems.org/blog/whatsapp/ to provide end-to-end encryption(Android non-group chat only for now). Apart from the fact that the client is still closed source and untrustable, they now seem to be in a better security situation than the other popular messaging apps.

What massive security holes?

For example, they store the message database on the shared mass storage partition (a.k.a. SD card), where it can be read by all installed applications.

That shouldn't happen in a properly sandboxed OS.

Wasn't it also true that the password for every account was a simple function of the phone number? Then they changed it, only to base in the IMEI instead.

I didn't look at it again so I don't know if they fixed it for real in the end.