I don't think just selling ads is sufficient; especially if it's not a service you can operate at a relatively low cost with a skeleton crew. I think it's probably either that or start charging for something. (Unless your goal isn't to ever make a profit, I suppose.)
Discord seemed to make it work (I think?) by combining an initial semi-skeleton crew approach with a freemium charging approach. They tried a few other things, but I think those efforts flopped.
Discord... definitely different. It's hard for me to believe that Nitro really can be paying for Discord but I guess it's possible?
1 million users * $10/month/user * 12 months/year = $120 million/year
That is close to the cited $130m annual revenue.
I don't know if it's primarily the format, or the different type of user compared to FB, but FB is the winner hands down.