The recommendations will appear in a new page within the Steam Community, which makes recommendations based upon your recent purchases and played games. The system also allows you to make recommendations to your friends and check out the most played games across your list of buddies.
This should help sell and promote some games, but it's a lot less invasive than asking users to tweet and Facebook share everything in their library. Valve isn't asking you to do that. At least, not yet...
I can't wait to spam my friends list :3