Slack has grown extremely fast based on the premise of improving internal communications at work. With "shared channels", they're hoping to "break down the walls", as their announcement says, and go beyond internal chat. I haven't tried it yet but I've heard good things about it.
The product team at Verizon Media continues improving Yahoo Mail more and more. We're big Gmail fans at ShuttleCloud but I've been using the Yahoo Mail app for my personal email and was quite impressed. Their 4.6 rating in Google Play shows I'm not the only one.
There are a few interesting things here. On the one hand, I have never tried Viber and I'm amazed it got so big. A lot of these messaging apps were started around the same time, and several found success even though the network effects in the messaging world encourage consolidation. I think that the number of people online grew so much in the last decade that it allowed several competing apps to coexist. One of the most authoritative bloggers in this space is Fred Wilson, who on his blog post about Kik's shutdown says that Kik still had 10mm monthly users and almost 5mm daily users, many of them teenagers.
I'm happy to see Google improving their internal chat tool so they can better compete with Slack and Microsoft Teams. Admittedly, they're years behind the leaders, but they have competitive advantages like their ability to integrate with Gmail. We'll be watching them closely.