$\text{STUMBLEUPON MINI CASE:}$ StumbleUpon Calls Itself a "Discovery Engine" Because, at One Time

$\text{STUMBLEUPON MINI CASE:}$ StumbleUpon calls itself a "discovery engine" because, at one time, 50.27% of all traffic from a top 10 list of social sites came from StumbleUpon. Their relatively small group of users (15 million) often out-refers the larger user base from Facebook (over 1.15 billion) . In fact, the influential users of StumbleUpon generate 1 billion page referrals per month. Their users, called "Stumblers", introduce other Stumblers and non-users to web content, providing a "map to an adventure you wouldn't otherwise have found out about." The influence of these Stumblers was deemed valuable by film marketers, who worked with StumbleUpon to promote films like Jurassic Park 3D and Evil Dead. Stumblers are young and highly engaged, so studios see the personalized search engine, which "learns" the preferences of its Stumblers, as a way to reach early influencers.
-In the STUMBLEUPON MINI CASE, the influencers are expert informal sources of information, also known as ____________________________.

A) Salespeople
B) Consumer Reports
C) Opinion leaders
D) Spokespeople
E) Buzz agents