They’re Alexa rank #16 globally - that they’re having trouble monetizing their userbase just means they have absolutely no idea what they’re doing.
Just because lots of people use something doesn't make them willing to pay for it, and it doesn't make a platform inherently monetizable.
Reddit is just as much of a profit driven company as Facebook or Twitter, I’m not sure why you’d think otherwise.
Because, among other things, they don't show annoying ad formats and don't demand real names and phone numbers from their users.
Not exactly a sustainable plan if you value using their service, and they’ve already got early investors pushing them to monetize.
If Reddit died tomorrow, we'd all shrug and find somewhere else to talk, just like the countless forums we used before Reddit that are no longer with us. Such is the danger for any social media: if all you provide is a commodity platform, you're only ever as valuable as the community you can build and whatever that community is willing to contribute, and ultimately you have very little control over those things.
Here's an old answer of mine comparing reddit's $7M to Instagram's $700M in 2014: https://www.quora.com/Why-are-reddits-advertising-revenues-o...
I dont know if they started selling this data directly. Last I spoke to ad people there, they had plans to build out their own tracking and analytics infrastructure, to improve things like geo-targeted ads, as their capabilities were lacking signficiantly.