If you think about similar products on the market (fb messenger, WhatsApp, even Line or Kakao Talk in Japan/Korea), this is quite unheard of, because WeChat is taking up everyone's time on the phone. Other apps just simply cannot divide a user's attention out of WeChat the app, on a smartphone.

And just because of that, most of the businesses and companies start to live off of wechat's user base and features. For example, there are subscription accounts where you get subscribed articles regularly so you don't need to build a news app anymore for content, and individual content producers can also publish just as easily, so the spiral gets deeper and deeper into the wechat system. And when payment started to come around on WeChat in 2014 (iirc), even Alipay is taking a hit, because now you literally can just keep WeChat open on your phone.

I think that, if you build an IM product and suddenly become the only app people ever use, this is just amazing.