But reddit isn't yet profitable, is it?
Nope, Conde Nast bought reddit a few years ago, and it was revealed last year (http://www.businessinsider.com/reddit-ceo-admits-were-still-in-the-red-2013-7) that it's still losing money. Interestingly, they were also trying new, what looked like targeted, Amazon ads just before this announcement. I kind of doubt it's specifically a selling announcement, and more focused on making reddit more attractive to AMAs for celebrities, as a user in that thread hypothesized.
It's all about user base, user base, user base. User base, to an investor, means eyes to whom you can sell to. Whether it be ads, Reddit gold, user information, or some other commodity, having the eyes and thoughts of millions means a great potential for revenue even if it hasn't been cashed in on yet. That's why Facebook got valued in the millions when it went public on the stock exchange before it even started to turn a dollar of profit.