My friends and I still use TS3. The audio quality and voice activation is better than Discord's, and the desktop app doesn't take ten fucking gigabytes of RAM to run.
The real question is: How in the world did Ventrillo continue to exist after TeamSpeak came along?
Vent was an object lesson in hostile UX. It sounded like shit, changing any kind of setting (even basic things like individual volumes) was a a gymnastics routine, and mics constantly clipped despite settings.
No joke last time I launched team speak was scary lmao. I didn't have my server anymore so my buddies and I joined a random one. As we were chilling and gaming random people joined in and called us the n word and then left over the course of our session lol definitely felt like a 360 CoD lobby
Still hosting TS as the primary place my friends record things because of the audio quality and especially reliability compared to Discord, but not so much for hangouts anymore. Got Mumble in the back pocket in case the licensing goes to crap though
I'm absolutely still using teamspeak. Nice and light, and it let's us run a soundboard plugin that let's you have unlimited length audio clips. I just wish they'd update the plugin to support the 64-bit version.
I still self host my TS3 for my nerd herd, and as an EvE online player (currently trying to win, but thats hard), you have to be fluent in all voip solutions as they all have different requirments and say a lot about your group.
Discord - small group, utilizing free services, may have an auth tool, used to keep in contact with people from old groups. Remember kids, if the product is free, you are the product
TS3 - mid-sized group (100-1000 players) requires a real IT team, will have an authentication system and generally will have their shit together. Ease of set up is handy, but admin user accounts can break servers.
Mumble - Welcome to the big leagues. (1K+ players) The resources you require now require resources in meat-space and are rather substantial. You need real IT security and people on a payroll. It will drive your admins nuts for about a week setting everything up, but once its done, you wont have to touch it again.
We use it for additional comms in foxhole, can't be in multiple channels at once and we've got about 8 groups that need to communicate between each other