Yahoo and Google are two interesting major content discovery platforms.
Facebook is not basically a content discovery platform but it is turning into one. Google+ is more specifically built for content discovery as it is a late entrant into the social media space and Google is the leader in content discovery. So if social media can become the content discovery mechanism, Google wanted a share of it. Google+ is a success so far.
Flipora is a new content discovery platform which will highlight content relevant to you on a continuous basis if you instal a plug in. YouTube is doing it on its platform.