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.
Yeah, this is the kicker. I think SO's primary intended audience is people clicking Google search results.
If I'm Googling something technical and I see Stack Overflow/Stack Exchange results, I always click those first, because I know I'm almost always going to attain the most helpful-information-per-unit-time that way. Even if an answer's many years old, it's usually going to be more helpful than most of the much more recent links, which are often just cookie cutter blogspam.
But the way to built that knowledge base is by answering peoples questions because the questions people ask indicate what is relevant to answer.
I only go to the public one a few times a day now, compared to being wired to it as a beginner :D
I think it's plausible Jack Dorsey maybe genuinely just didn't want to sign that deal with the devil. I know I wouldn't want to if I made Twitter. (I have no idea if that's what happened, of course. Maybe he wanted to but couldn't find a good way to achieve it.)
If your service's business strategy is "1) acquire hundreds of millions of users and charge them all $0.00 per year for many years, 2) acquire lots of expensive infrastructure and employees to support the service, 3) ???, 4) profit", it's not going to be easy.