From my observation it's: 1. Promoted posts, inline but called out to some extent. 2. Job posts. In HN's case, these are combined but there is also the "Jobs"

From my observation it's: 1. Promoted posts, inline but called out to some extent. 2. Job posts. In HN's case, these are combined but there is also the "Jobs" page in the nav up top. 3. Regular ad network stuff as well. (Example Adsense, Carbon, etc) 4. Merch for sale 5. Paid mobile app In the case of Reddit, they also do: Gold status which unlocks a few non important things