During an interview I had at Discord recently the lead engineer mentioned they had exceeded 130 million users. Though, there was also a recent bump by almost 70 million people that left Skype and joined Discord in the news recently as well. He told me that Skype is the 800 pound gorilla to beat and that they have around 300 million users or something. They're definitely gunning for them, and Microsoft has changed Skype so much that they've alienated much of their user base in addition to having lots of security flaws. Discord touts their security, two factor auth, IP address hiding, plus having all the features of Skype in addition to the concept of slack-like chat rooms. More than just gamers are starting to use it.
https://variety.com/2018/gaming/news/discord-turns-three-120...
At the moment, Discord is ad-free and other than Nitro subscriptions which let you have animated profile images and use emoticons in any server, which he admitted was an almost unnoticeable revenue stream at the moment, they don't really have a way to monetize the platform. When I asked how they were planning to generate revenue and whether my job would be secure he said they haven't settled on any ideas yet but have a few bouncing around that he wouldn't tell me.
He mentioned that once they reach 600 million users they consider that the break-even point where they've succeeded as a platform and can start considering putting possible plans into action. My guess is that advertising will play a big part, and possibly various other techniques like microtransactions, platform integrations, affiliates, partnership deals, content distribution (movies, music, games), etc. It is probably the break point at which you can afford to anger a percentage of your users in exchange for enough revenue to establish themselves as a self-sufficient organization.