Valuation is usually based on potential revenue, not actual revenue. A $2 billion valuation would have been about $50 per user. It seems reasonable Stack Exchange should be able to make that much money per user somehow over the entire future existence of the company... if it's run well. Which doesn't seem to be happening from where I'm sitting.

I'm unsure of many people know that StackOverflow also had enterprise offerings. Our company has their own StackOverflow instance with very specific content to our tech stacks.