In Singapore, where there are not many people using J2ME phones with a 3G data plan, this is why:
Facebook - not everyone has a Facebook account (think parents, young kids), you're not Facebook friends with everyone you know, and you don't _want_ to be Facebook friends with everyone you need to contact. I think they have been trying to change this on Facebook Messenger, but still, you need a Facebook account, and that's a line that many people refuse to cross on principle.
Hangouts - not everyone has Google account, because they use another email provider (Hotmail, Yahoo, ISP), and don't want to get "a Gmail account" just to use it. Granted, this might work great for Android users, but in practice, nobody uses this here (except maybe for multi-party video conferencing). Practically don't see iPhone users on Hangouts either; it is very much associated with Android (and hiding in the background, many people don't even know they're logged in because they don't use it, and nobody uses it to contact them).
iMessage - doesn't work on Android. Yes, there are many iPhones in the world, but you are incredibly selective about who you need to contact if they all have iMessage. But granted, this works quite transparently between iPhones.
Now, iMessage (and Viber) is probably the closest to the 0-step-to-add-contact way of WhatsApp. Because there's no process to do that, there's no contact-request 'approval' required from the other side, there's no friction. You add someone to your phone book, they show up in WhatsApp, you send a message or add them to a group. That's it.
It is a lot easier to teach folks to use WhatsApp just because of this. It's about as complicated to use as your platform's built-in SMS client. It is a huge contrast with the 101 features of WeChat, and to a lesser extent, LINE. WeChat is _huge_ with the PRC community here, but that's about it. Everyone else is on WhatsApp.
Viber is probably the next in line, but due to WhatsApp's network effects, it just didn't take off. Sure, it has internet voice calls, but it's known for being a little flakey on the call quality, and by default its notifications can be a bit annoying. WhatsApp never pops things up unless someone sent you a message.
Facebook - not everyone has a Facebook account (think parents, young kids), you're not Facebook friends with everyone you know, and you don't _want_ to be Facebook friends with everyone you need to contact. I think they have been trying to change this on Facebook Messenger, but still, you need a Facebook account, and that's a line that many people refuse to cross on principle.
Hangouts - not everyone has Google account, because they use another email provider (Hotmail, Yahoo, ISP), and don't want to get "a Gmail account" just to use it. Granted, this might work great for Android users, but in practice, nobody uses this here (except maybe for multi-party video conferencing). Practically don't see iPhone users on Hangouts either; it is very much associated with Android (and hiding in the background, many people don't even know they're logged in because they don't use it, and nobody uses it to contact them).
iMessage - doesn't work on Android. Yes, there are many iPhones in the world, but you are incredibly selective about who you need to contact if they all have iMessage. But granted, this works quite transparently between iPhones.
Now, iMessage (and Viber) is probably the closest to the 0-step-to-add-contact way of WhatsApp. Because there's no process to do that, there's no contact-request 'approval' required from the other side, there's no friction. You add someone to your phone book, they show up in WhatsApp, you send a message or add them to a group. That's it.
It is a lot easier to teach folks to use WhatsApp just because of this. It's about as complicated to use as your platform's built-in SMS client. It is a huge contrast with the 101 features of WeChat, and to a lesser extent, LINE. WeChat is _huge_ with the PRC community here, but that's about it. Everyone else is on WhatsApp.
Viber is probably the next in line, but due to WhatsApp's network effects, it just didn't take off. Sure, it has internet voice calls, but it's known for being a little flakey on the call quality, and by default its notifications can be a bit annoying. WhatsApp never pops things up unless someone sent you a message.