sigh... you guys make it so hard to stay away
There's no ads on reddit, its entirely funded by voluntary donations. no one has to pay anything at any time. People can promote a comment by "giving them gold" basically paying $5 to say, I really like what this guy said, and think more people should see it, so its likely to rise higher when you filter by "best" or "top"
As for the public decency/park analogy argument, there are decency rules on reddit too, they weren't well defined before, they're getting better definition now but are still emerging.
That's not true at all. There are ads on reddit, just not a lot because they're advertiser kryptonite. They're trying to change this in an effort to become profitable (with Yishan claiming the board is pressuring them to change faster).