iirc, they used to be curated, over time organic features were added to promote popular games, and the social update brought the "Recommended for you" pages.
You can tell explicit placement still goes on, as games that are in the news pop up, so do preorders of big titles, etc.
If I had to imagine, how it works now is theres several buckets of candidates, such as popular new releases, curated features (IE "New DLC Available" or "Preorder Now", "New on Steam" etc), and Recommended For You, and a certain amount of titles are shown from each bucket every time the front page is shown.
You can tell explicit placement still goes on, as games that are in the news pop up, so do preorders of big titles, etc.
If I had to imagine, how it works now is theres several buckets of candidates, such as popular new releases, curated features (IE "New DLC Available" or "Preorder Now", "New on Steam" etc), and Recommended For You, and a certain amount of titles are shown from each bucket every time the front page is shown.