Additionally, there are numerous search engines, unlike Facebook and Twitter which have effective monopolies over social media news sites.
In the case of social networks, Zuckerberg is 100% on the money on this one. Anecdotally, I've noticed my Twitter and Facebook feeds are extremely diverse in opinion and philosophy. The articles linked to and discussions had on each of my home pages are wildly different than what I would find if I were only searching through the news sites I habitually visit.